Summary:
This enables fetching blobs from the LFS server. For now, this is limited to
fetching them, but the protocol specify ways to also upload. That second part
will matter for commit cloud and when pushing code to the server.

One caveat to this code is that the LFS server is not mocked in tests, and thus
requests are done directly to the server. I chose very small blobs to limit the
disruption to the server, by setting a test specific user-agent, we should be
able to monitor traffic due to tests and potentially rate limit it.

Reviewed By: DurhamG

Differential Revision: D20445628

fbshipit-source-id: beb3acb3f69dd27b54f8df7ccb95b04192deca30