Add ucumUnit keyword support (Units extension)

[clemensv]

Summary

The Units extension now includes a new ucumUnit keyword that allows annotating numeric values with UCUM (Unified Code for Units of Measure) notation alongside the existing unit keyword.

Required Changes

1. Add ucumUnit to the Units extension keyword list in schema validators
2. Validate that ucumUnit values are strings (same as unit)
3. Allow ucumUnit to coexist with unit on the same schema element

Context

See units PR #5 for the spec text.

Affected SDKs

All language SDKs: TypeScript, Python, .NET, Java, Go, Rust, Ruby, Perl, PHP, Swift, C