Allow passing auth plugin as a parameter

This further separates the concept of session and auth plugin so that
the session can be a completely standalone transport layer.

This is similar to how requests handles things. You can install an auth
plugin on the session object and then everything that works through the
session will be authenticated or you can specify the auth plugin per
request and only authenticate that request.

Change-Id: If4ed5af831cc35e259d9f963062261819f08a9d5
Merge "Allow passing auth plugin as a parameter"