Support alpha and beta API access

[rjschwei]

The discovery API (https://github.com/google/google-api-python-client) provides access to alpha and beta apis. THis implementation is being replaced by individual component implementations, for example https://github.com/googleapis/google-cloud-python. However the components do not provide access to alpha and beta features effectively removing functionality.

Especially for those of us who need beta features quite frequently due to us publishing images to GCE the loss of functionality puts us in a bind. We either have to continue to use the toolchain marked "maintenance mode" or move to the replacement tools with a loss of functionality. The later has impact on our joint customers as we cannot set certain feature flags for our images.

[parthea]

See feature request in public issue tracker https://issuetracker.google.com/issues/335426676