This changeset allows Workload Identities to authenticate to all the RPCs that
support HTTP API endpoints, for use with PR #15864.
* Extends the work done for pre-forwarding authentication to all RPCs that
support a HTTP API endpoint.
* Consolidates the auth helpers used by the CSI, Service Registration, and Node
endpoints that are currently used to support both tokens and client secrets.
Intentionally excluded from this changeset:
* The Variables endpoint still has custom handling because of the implicit
policies. Ideally we'll figure out an efficient way to resolve those into real
policies and then we can get rid of that custom handling.
* The RPCs that don't currently support auth tokens (i.e. those that don't
support HTTP endpoints) have not been updated with the new pre-forwarding auth
We'll be doing this under a separate PR to support RPC rate metrics.
