Refactor OAuth so that it doesn't require private keys when we aren't signing anything

Now, we can safely use bearer tokens or "none" alg without guice crapping out. I also refactored naming of things.

As always in this api, there's more shoveling to do, but this is certainly a step forwards cognitively and also from a tech debt pov.

replaces #87

 * Refactor OAuth so that it doesn't require private keys when we aren't signing anything.
 * Rename constant `serviceAccountCredentials` to `p12PrivateKeyCredentials` as this better describes the credential value.
Not an enum as this is spec is a draft still. Though it may not hurt as
this is input only. That said doing so would complicate other data
structures to now have mixed type.. I think it is ok. Good question though!