feat(tools): enum, regex, referential type feature for csharp codegen

[ragi-dayananda]

Changes

• Add regex annotation for csharp codegen when regex configured for a field
• UUID nullable type bug fix
• Make optional string properties as nullable string.
• Custom enum value support with AcceptedValue decorator
• Handle referential type (-->) in csharp codegen

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary
• Merging to main from fork:branchname

[dselman]

I worry that we might be using an externalFieldType here...

[ragi-dayananda]

yes, we might. But external type can be UUID scalar type. We don't want to add regex decorator for UUID.

[dselman]

Looks great!