ICU-23018 Add unit conversion API with comprehensive tests (C++)

[younies]

Description:

• Introduced UnitsConverter, CompoundUnitsConverter, and related classes to facilitate unit conversions.
• Implemented a new test suite in UnitsConverterTest to validate conversion accuracy across various units, including length, speed, and temperature.
• Enhanced UnitsConverter with cloning capabilities.
• Added builder pattern support for CompoundUnitsConverter to streamline the conversion process.

Checklist

• Required: Issue filed: ICU-23018
• 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