Add conversion support to MeasureUnit

[younies]

Description

This update introduces the followings

• The ConversionInfo class to the MeasureUnit, which encapsulates conversion details such as conversion rates and offsets to the base unit.
• Additionally, a new method, convertFrom, is implemented to facilitate conversions between MeasureUnits, ensuring proper handling of mixed units and convertibility checks.
• The constructor for MeasureUnit is also updated to accommodate the new conversion information.
• The internal measure units would be updated to include the conversion info.

TODO: Please describe your changes here.

TODO: Please read the following on ICU Contributing, and then delete these instructions.

Thank you for your pull request!

• For general info on contributing: https://github.com/unicode-org/icu/blob/main/CONTRIBUTING.md
• Associating PRs with Jira issues
  • We require each pull request to be associated with a Jira issue.
  • Reuse existing issues for minor changes:
    • ICU 78 docs minor fixes: ICU-23055 — User Guide & API docs typos etc., and version updates (e.g., dependabot for User Guide)
    • ICU 78 code warnings/version updates: ICU-23054 — Fix compiler warnings. Update versions of code-related dependencies (e.g., dependabot).
• Contributors license agreement (CLA):
  • You will be automatically asked to sign the CLA before the PR is accepted.
  • To sign the CLA: https://cla-assistant.io/unicode-org/icu
  • For terms of use and license, see https://www.unicode.org/copyright.html

TODO: Fill out the checklist below.

Checklist

• Required: Issue filed: ICU-NNNNN
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[macchiati]

Looking pretty good

[macchiati]

The cache of measure units should use ConcurrentHashMaps, not synchronization.

[younies]

I didn’t fully understand the comment — could you please explain it in more detail?