Install `icu` in our docker images

[anoadragon453]

#14464 added icu as a new, optional native dependency to improve user directory search for non-English languages. If the python module is installed, then the new user search code is used, otherwise it will fall back to the old code.

It would be nice to include this in our docker images by default as it allows for enhanced functionality. Really all this means is:

1. Install libicu-dev
2. Ensure pyicu is installed via poetry

[ankit-pn]

it has been done in #14712

[DMRobertson]

I sanity checked this as follows:

$ docker exec -it synapse python -ic "import icu"
>>> icu
<module 'icu' from '/usr/local/lib/python3.9/site-packages/icu/__init__.py'>
>>> dir(icu)
['AlphabeticIndex', 'AnnualTimeZoneRule', 'BasicTimeZone', 'Bidi', 'BidiTransform', 'BreakIterator', 'BytesTrie', 'BytesTrieBuilder', 'BytesTrieIterator', 'BytesTrieState', 'Calendar', 'CanonicalIterator', 'CaseMap', 'Char', 'CharacterIterator', 'CharsetDetector', 'CharsetMatch', 'ChoiceFormat', 'CollationElementIterator', 'CollationKey', 'Collator', 'CompactDecimalFormat', 'ConstrainedFieldPosition', 'CurrencyAmount', 'CurrencyPluralInfo', 'CurrencyPrecision', 'CurrencyUnit', 'DateFormat', 'DateFormatSymbols', 'DateInterval', 'DateIntervalFormat', 'DateIntervalInfo', 'DateRuleType', 'DateTimePatternGenerator', 'DateTimeRule', 'DecimalFormat', 'DecimalFormatSymbols', 'DictionaryBasedBreakIterator', 'Edits', 'EditsIterator', 'FLOATING_TZNAME', 'FieldPosition', 'FilteredNormalizer2', 'FloatingTZ', 'Format', 'Formattable', 'FormattedDateInterval', 'FormattedList', 'FormattedNumber', 'FormattedNumberRange', 'FormattedRelativeDateTime', 'FormattedValue', 'ForwardCharacterIterator', 'FractionPrecision', 'GenderInfo', 'GregorianCalendar', 'ICUError', 'ICU_MAX_MAJOR_VERSION', 'ICU_VERSION', 'ICUtzinfo', 'IDNA', 'IDNAInfo', 'ImmutableIndex', 'IncrementPrecision', 'InitialTimeZoneRule', 'IntegerWidth', 'InvalidArgsError', 'ListFormatter', 'Locale', 'LocaleBuilder', 'LocaleData', 'LocaleMatcher', 'LocaleMatcherBuilder', 'LocaleMatcherResult', 'LocalizedNumberFormatter', 'LocalizedNumberRangeFormatter', 'Measure', 'MeasureFormat', 'MeasureUnit', 'MessageFormat', 'MessagePattern', 'MessagePattern_Part', 'NoUnit', 'Normalizer', 'Normalizer2', 'Notation', 'NumberFormat', 'NumberFormatter', 'NumberRangeFormatter', 'NumberingSystem', 'PY_VERSION', 'ParsePosition', 'PluralFormat', 'PluralRules', 'Precision', 'PythonReplaceable', 'RegexMatcher', 'RegexPattern', 'Region', 'RelativeDateTimeFormatter', 'Replaceable', 'ResourceBundle', 'RuleBasedBreakIterator', 'RuleBasedCollator', 'RuleBasedNumberFormat', 'RuleBasedTimeZone', 'Scale', 'ScientificNotation', 'Script', 'SearchIterator', 'SelectFormat', 'Shape', 'SimpleDateFormat', 'SimpleFormatter', 'SimpleTimeZone', 'SpoofChecker', 'StringCharacterIterator', 'StringEnumeration', 'StringSearch', 'StringTrieBuilder', 'TimeArrayTimeZoneRule', 'TimeRuleType', 'TimeUnit', 'TimeUnitAmount', 'TimeUnitFormat', 'TimeZone', 'TimeZoneRule', 'TimeZoneTransition', 'Transliterator', 'UAcceptResult', 'UAlphabeticIndexLabelType', 'UBiDiDirection', 'UBiDiMirroring', 'UBiDiOrder', 'UBiDiReorderingMode', 'UBiDiReorderingOption', 'UBidiPairedBracketType', 'UBlockCode', 'UCalendarAMPMs', 'UCalendarDateFields', 'UCalendarDaysOfWeek', 'UCalendarMonths', 'UCharCategory', 'UCharCharacterIterator', 'UCharDirection', 'UCharNameChoice', 'UCharsTrie', 'UCharsTrieBuilder', 'UCharsTrieIterator', 'UCharsTrieState', 'UCollAttribute', 'UCollAttributeValue', 'UCollationResult', 'UCurrNameStyle', 'UCurrencySpacing', 'UCurrencyUsage', 'UDateAbsoluteUnit', 'UDateDirection', 'UDateFormatBooleanAttribute', 'UDateRelativeDateTimeFormatterStyle', 'UDateRelativeUnit', 'UDateTimePatternConflict', 'UDateTimePatternField', 'UDateTimePatternMatchOptions', 'UDisplayContext', 'UDisplayContextType', 'UFieldCategory', 'UGender', 'UGraphemeClusterBreak', 'UHangulSyllableType', 'UIndicPositionalCategory', 'UIndicSyllabicCategory', 'UJoiningGroup', 'ULineBreak', 'UListFormatterField', 'ULocDataLocaleType', 'ULocMatchDemotion', 'ULocMatchDirection', 'ULocMatchFavorSubtag', 'ULocaleDataDelimiterType', 'ULocaleDataExemplarSetType', 'UMatchDegree', 'UMeasureUnitComplexity', 'UMeasurementSystem', 'UMemory', 'UMessagePatternApostropheMode', 'UMessagePatternArgType', 'UMessagePatternPartType', 'UNICODE_VERSION', 'UNORM_INPUT_IS_FCD', 'UNormalizationCheckResult', 'UNormalizationMode', 'UNormalizationMode2', 'UNumberCompactStyle', 'UNumberDecimalSeparatorDisplay', 'UNumberFormatAttribute', 'UNumberFormatFields', 'UNumberFormatRoundingMode', 'UNumberFormatStyle', 'UNumberGroupingStrategy', 'UNumberRangeCollapse', 'UNumberRangeIdentityFallback', 'UNumberRangeIdentityResult', 'UNumberSignDisplay', 'UNumberUnitWidth', 'UObject', 'UProperty', 'UPropertyNameChoice', 'URBNFRuleSetTag', 'URegexpFlag', 'URegionType', 'URelativeDateTimeFormatterField', 'URelativeDateTimeUnit', 'UResType', 'URestrictionLevel', 'USET_ADD_CASE_MAPPINGS', 'USET_CASE_INSENSITIVE', 'USET_IGNORE_SPACE', 'UScriptCode', 'UScriptUsage', 'USearchAttribute', 'USearchAttributeValue', 'USetSpanCondition', 'USpoofChecks', 'UStringTrieBuildOption', 'UStringTrieResult', 'UTimeUnitFields', 'UTimeUnitFormatStyle', 'UTransDirection', 'UTransPosition', 'UVerticalOrientation', 'UWordBreak', 'UWordBreakValues', 'U_COMPARE_CODE_POINT_ORDER', 'U_COMPARE_IGNORE_CASE', 'U_EDITS_NO_RESET', 'U_FOLD_CASE_DEFAULT', 'U_FOLD_CASE_EXCLUDE_SPECIAL_I', 'U_OMIT_UNCHANGED_TEXT', 'U_TITLECASE_ADJUST_TO_CASED', 'U_TITLECASE_NO_BREAK_ADJUSTMENT', 'U_TITLECASE_NO_LOWERCASE', 'U_TITLECASE_SENTENCES', 'U_TITLECASE_WHOLE_STRING', 'UnicodeFilter', 'UnicodeFunctor', 'UnicodeMatcher', 'UnicodeSet', 'UnicodeSetIterator', 'UnicodeString', 'UnlocalizedNumberFormatter', 'UnlocalizedNumberRangeFormatter', 'VERSION', 'VTimeZone', '__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__path__', '__spec__', '_icu_']

Ensure pyicu is installed via poetry

Note that this should be done for free: we effectively poetry install --extras all and user-search was added to all.

[DMRobertson]

it has been done in #14712

In particular https://github.com/matrix-org/synapse/pull/14712/files#diff-f34da55ca08f1a30591d8b0b3e885bcc678537b2a9a4aadea4f190806b374ddcR170