Retry logic for all GET's in GithubClient

[dylan-smith]

We added retry logic to all ADO API calls directly in the AdoClient class. We should do the same thing in the GithubClient class. This is specifically to deal with transient networking type errors that we occasionally see causing flakiness in our integration tests (and no doubt impact customers also).

[dylan-smith]

Thinking about this more, we should probably retry on any error in every GET (both AdoClient and GithubClient). Retrying on a 401 or 404 seems kinda wasteful, but I've seen enough random transient errors at this point, that I think it's best to just retry on every error on a GET call.