xds/gcp_authn: implement GCP Authentication filter (gRFC A83). (#9119)

This PR implements the xDS GCP Authentication HTTP filter for gRPC-Go,
as specified in [gRFC
A83](https://github.com/grpc/proposal/blob/master/A83-xds-gcp-authn-filter.md).

The implementation provides a mechanism for attaching service account
identity JWT tokens as gRPC call credentials on GCP, featuring a
two-level caching strategy (filter-level cache of credentials objects
and credential-level cache of tokens) to minimize latency and metadata
server load.

RELEASE NOTES:
* xds: add support for the GCP Authentication HTTP Filter. It
automatically fetches and attaches GCP Service Account Identity JWT
tokens to outgoing RPCs. This feature is currently gated behind the
`GRPC_EXPERIMENTAL_XDS_GCP_AUTHENTICATION_FILTER` environment variable.