Support for multiple api keys

[elgreg]

When rotating API keys, it is very useful to have an application that can support both the old and new key at the same time so that various services can be updated one at a time.

I spoke a bit with @jonwinton about this and he suggested potentially having a clay app.js pass the apikeys that it wants to amphora when it is setup.

This is something like what that might look like (but branched off of amphora v4.3.0): slategroup/amphora@v4.3.0...slategroup:passing-apikeys-support

[pfeytser]

This falls under auth, which is a topic we're tackling in a few sprints.