Add the metadata field to create DependencyGraphSnapshot

[eyalk007]

Fixes: #3635.

This PR adds support for the metadata field in the DependencyGraphSnapshot struct, enabling users to include custom information when submitting dependency snapshots to GitHub's dependency graph API.

I have added comprehensive test coverage including JSON serialization validation.

This implementation follows the official GitHub API specification. The field type is map[string]any matching the API's object type. It has constraints of being limited to 8 keys with scalar values as documented.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.33%. Comparing base (83ae84c) to head (2bb5bc6).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3640   +/-   ##
=======================================
  Coverage   91.33%   91.33%           
=======================================
  Files         184      184           
  Lines       16174    16174           
=======================================
  Hits        14773    14773           
  Misses       1227     1227           
  Partials      174      174           

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[gmlewis]

Thank you, @eyalk007!
LGTM.
Merging.