Enable GitHub Realtime authentication

[rosecodym]

Description:

GitHub Realtime needs to be able to authenticate. This PR adds relevant fields to the relevant protobuf message, and exports/tweaks some stuff from the github source package to permit its (eventual) use by the GitHub Realtime implementation:

• The Connector interface and its constructors are now exported
• Connector.Clone now accepts additional clone arguments
• The github-specific connection construction has been moved into github.go
  • The injected rate limit handler no longer accepts error reporters that it never uses

I also added an API endpoint field to the GitHub Realtime proto message, because we need that too and I was here.

Checklist:

• Tests passing (make test-community)?
• Lint passing (make lint this requires golangci-lint)?