xds: Envoy should support its built-in LB policies as extensions

LB extensibility was originally described in #5598. The infrastructure for this has been implemented, but there are a few missing pieces. Here's the relevant history:
- A new field for LB policy extensibility was added to xDS in #7744.
- The extension mechanism was added to Envoy in #17400.
- The semantics of the xDS field were changed slightly in #18419.
- xDS extension protos were added for ROUND_ROBIN and RING_HASH in #19517 and #20423, but Envoy does not yet actually implement configuring those policies as extensions.

I think the remaining work here is:
- Add extension protos for all built-in LB policies.
- Implement actually configuring these policies via the extension protos.

Once this is done, we can consider whether we want to deprecate the existing enum for configuring built-in policies.