Implement cache provider for Github source plugins

[Marak]

Ideally, we wouldn't want to be requesting the service's source from Github on every request.

It would be better if the plugins for Github remote sources were able to accept a cache provider ( similar to how the rate limiter works ) in order to provide the option of caching the source code ( for fast retrieval ).

We'd also want to accept an option to indicate that the cache should be invalidated ( on request ).