Add support for Metadata

[ajay-k]

Fixed missing metadata field type in AdditionalFieldType enum that was supported by the API but not available in the SDK.
Changes

• Added METADATA enum value with "Json"(name = "metadata") annotation
• Updated documentation to include metadata in supported field types
• Added tests for serialization and API integration

License

I confirm that this contribution is made under the terms of the MIT license and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.57%. Comparing base (ce3598c) to head (1de3dbf).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #284   +/-   ##
=========================================
  Coverage     80.57%   80.57%           
  Complexity      333      333           
=========================================
  Files            36       36           
  Lines          1014     1014           
  Branches         88       88           
=========================================
  Hits            817      817           
  Misses          138      138           
  Partials         59       59           

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