A blocking issue for #9301 is that we're using the v3alpha DiscoveryRequest message for REST subscriptions. This is broken, since we're placing deprecated fields on the wire in #9301 (which introduces the first v2 vs. v3 distinction in the transport protocol).

We should scrub envoy_hidden_ fields prior to sending out both gRPC and REST subscriptions discovery requests when connecting to a v3 endpoint (to avoid unknown fields), and downgrade the message to v2 when connecting to a v2 endpoint (to avoid shadow names for deprecated fields).