Add Metadata field to DependencyGraph* structs

[eranturgeman]

Fixes: #3652.

This PR enhances DependencyGraphSnapshotManifest and DependencyGraphSnapshotResolvedDependency structs with Metadata property, following GitHub documentation.
This property enables to provide user-defined metadata to store domain-specific information limited to 8 keys with scalar values.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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##             master    #3653   +/-   ##
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  Coverage          ?   91.25%           
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  Files             ?      184           
  Lines             ?    16311           
  Branches          ?        0           
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  Hits              ?    14884           
  Misses            ?     1245           
  Partials          ?      182           

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[gmlewis]

Thank you, @eranturgeman!
LGTM.
Merging.