[C++] Support parsing of "2020-01" into `2020-01-01`

[asfimport]

Originated as part of the review for the ARROW-16407 PR.

Currently we get a surprising output when parsing of %Y-%m strings such as "2020-01" - see reprex below.

library(arrow, warn.conflicts = FALSE)

call_function(
  "strptime",
  Array$create("2020-01"),
  options = list(
    format = "%Y-%m",
    unit = 0L
  )
)
#> Array
#> <timestamp[s]>
#> [
#>   2019-12-31 00:00:00
#> ]

I'm not sure if this would be useful somewhere else, but we could definitely use this to remove some workarounds in the R bindings.

Reporter: Dragoș Moldovan-Grünfeld / @dragosmg
Watchers: Rok Mihevc / @rok

Related issues:

• [R] [C++] Should strptime support a partial format? (relates to)

_{Note: This issue was originally created as ARROW-16627. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
This is a bug more than a feature.

[asfimport]

Rok Mihevc / @rok:
Seems like strptime has some sort of consistent behaviour here:

import pyarrow as pa
sarr = pa.array(["2022-01-00", "2022-01-01"], type=pa.string())
pa.compute.strptime(sarr, format="%Y-%m-%d", unit="s")

<pyarrow.lib.TimestampArray object at 0x121b98ac0>
[
  2021-12-31 00:00:00,
  2022-01-01 00:00:00
]

Definitely unexpected enough to be a bug.