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.