feat(tools): csharp codegen nullable type support

[ragi-dayananda]

Signed-off-by: [email protected] [email protected]

Closes #

Changes

• Nullable fields support for csharp codegen. Considering optional fields as nullable fields in csharp codegen.
• Add System. before Guid to avoid namespace collision or to prevent importing the System namespce.

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