Add gcp auth & rename s3_auth plugin to origin_server_auth

[jasmine-nahrain]

Adding GCP authentication to existing s3_auth plugin. As this makes the s3_auth plugin no longer exclusively s3. I have renamed the plugin to origin_server_auth. This better reflects the full functionality of the plugin.

I have changed the parameter version from an int to a string. Instead of specifying 2 or 4 for the version, you must specify awsv2, awsv4 or gcpv1. This should allow adding more versions/auth services easier. The old versioning is now deprecated and should be removed in a later release.

s3_auth is now deprecated.