Add GitHub PR comment when PR Validation is complete

[JoeRobich]

When a Roslyn Validation PR is created, we should comment a link to the VS PR on the Roslyn PR being validated. This would be performed by new logic in the OneOffInsertion script that is used to generate the insertion PRs.

[davidwengier]

I wonder if it would be useful, especially for community contributors who can't see the insertions, to also have the comment include the feed URL and version information for the packages that are published to the general-testing nuget feed?

[JoeRobich]

Possible verbiage:

A validation build has been started for this PR. Click here to view (Microsoft only)

Package Information:

Version	Feed
3.10.0-1.21078.40	https://pkgs.dev.azure.com/dnceng/public/_packaging/general-testing/nuget/v3/index.json