src-expose is first class

Created by: keegancsmith

I am going to tackle a few quick wins in making src-expose feel first class. This issue will track them.

• git protocol v2. For perf and solves #11707
• better external service integration
• integrate just serve into src-cli #9243 (closed)
• list option for interactive debugging
• test src-cli docker image
• document and publish src git-serve

This should help a lot with the current pains listed in PD 10: Support for non-Git VCS. However, solving non-Git VCS in general is likely a larger project.

cc @christinaforney @dadlerj