Refactor service auth from Java to configuration

[rtannenbaum]

@olsona is in the progress of migrating service auth handlers to use 2 standard auth handlers we will add to the core framework (oauth1 and oauth2) and currently has service-specific information (auth URLs, etc) in java classes. This is in progress in https://github.com/google/data-transfer-project/issues/553.

In our maintainers meeting yesterday we discussed that the service-specific info should eventually be moved to configuration rather than java classes.

@jimmarino had some specific ideas for how to do this, I'll let him chime in with more details on that.

[lisad]

We reviewed this in maintainers meeting today. We think the task is to make the OAuth config code to be more abstract about what the "secret" is. It may not only be a string, it could be a JWT or a class for even greater abstraction and pluggability.