xds: send no-overprovisioning client capability and populate user_agent fields

[markdroth]

This changes the xds client to send the new client capability introduced to xds in envoyproxy/envoy#10136. We also now populate the new user_agent_name and user_agent_version fields.

This change is a bit bloated, because it required updating the envoy-api, protoc-gen-validate, and udpa submodules and regenerating all of the upb codegen files. The only substantive changes are in the following files:

• xds_api.{h,cc}
• xds_client.{h,cc}
• xds_end2end_test.cc

[donnadionne]

suggesting passing request into PopulateNode and move
envoy_api_v2_core_Node* node_msg =
envoy_api_v2_DiscoveryRequest_mutable_node(request, arena.ptr());
into PopulateNode in all 4 places

[markdroth]

I thought about that, but it won't work, because in XdsApi::CreateLrsInitialRequest(), the outer message is a LoadStatsRequest instead of a DiscoveryRequest.

[gnossen]

LGTM on build changes.

[markdroth]

Known issues: #22174 #21746 #20677 #22137 #13379

The ruby failure and the cloud-to-cloud interop failure are both infrastructure timeouts.

I am not going to squash commits here as we usually do, since that will make it easier to backport this to the 1.28 release branch in the wake of #21929.