Add Relations extension support (identity, relations keywords)

[clemensv]

Summary

The new JSON Structure: Relations extension defines the following keywords that need SDK support:

• \identity\ — declares which properties uniquely identify instances
• 
  elations\ — declares inter-type relationships
• \ argettype\ — specifies the target type of a relation
• \cardinality\ — \single\ or \multiple\
• \scope\ — JSON Pointer(s) to collections containing target instances
• \qualifiertype\ — additional properties qualifying the relationship

Required Changes

1. Add \JSONStructureRelations\ to the extension gate lists (currently only Import, AlternateNames, Units, ConditionalComposition, Validation are listed)
2. Add schema validation for the relations keywords (type checking, constraint enforcement)
3. Add the keywords to reserved keyword lists

Affected SDKs

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