GitHub timestamp support for release-age cooldown enforcement

[LalatenduMohanty]

Summary

GitHubTagProvider currently skips cooldown enforcement with a warning because GitHub's tag list API does not return timestamps. Add timestamp retrieval so cooldown can be enforced for GitHub-sourced packages.

Until this is implemented, GitHub-sourced packages skip cooldown with a warning (supports_upload_time = False). No manual min_release_age: 0 overrides are needed in the interim.

Design

See Phase 3 in the release cooldown proposal.

Ref: #877

[ryanpetrello]

Just a note for whoever implements this, @tiran left some good commentary on approach over on the original issue for cooldown functionality:

#877 (comment)

#877 (comment)

We'll need to analyze what GitHub makes available to us and consider the performance implications of potentially needing to make many API calls to determine release dates for tags.

If we end up implementing something using GitHub's GraphQL API, I believe that requires authentication, so we may have to consider how that ties into the expectation of GITHUB_TOKEN being set.