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.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (master@6dda213). Learn more about missing BASE report.
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[gmlewis]

Thank you, @eranturgeman!
LGTM.
Merging.