repo-updater: Parameterise http.Client of GithubSources
Created by: tsenart
This change-set parameterises the http.Client used by GithubSources to enable use to more easily test these Sources without huge setup and run-time costs.
It introduces a helper package called httpcli which faciliates the construction of a fully featured http.Client, wrapped with Middleware when necessary.
This is a pre-cursor PR to #2649 which will need extensive tests on GithubSource.