Adding a createFromFile method to AuthCredentials

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).

I would definitely prefer createFor(stream) rather than createFor(File) as the latter is not supported in some systems (such as AE).

Do you think this issue is a dup of #3?

In any case, I think if we add it it should only support a json content (so maybe method name should state it).

The reasons I don't think we should support p12 in this method are:

User can change their password (so we need to ask for that).
I am not sure we can extract the service_acount from it.
Seems to be deprecated.

Ooops sorry I missed #3. Yes this is a duplicate :(

I prefer createFor(stream) as well as it supports more uses. And I also agree that we can live without P12:

AFAIK it does not contain service_account information
It is supported just for backward compatibility with code that already used P12

GoogleCredential.createFor also supports only JSON format and states it in the javadoc. If you want to be more precise we can name it createForJson() but it kind of breaks the naming convention.