Adding a createFromFile method to AuthCredentials #179
Description
We now have a createFor(String account, PrivateKey privateKey) to create AuthCredentials, what about adding a createFor(File) for creating AuthCredentials from a service account JSON file?
Notably GoogleCredential already has fromStream that reads credentials from a stream of the JSON file. If you prefer, we can go for createFor(Stream) rather than createFor(File).