Introduce grpc-1.30+ compatible client/v3/naming API.

This is not yet implementation, just API and tests to be filled
with implementation in next CLs,
tracked by: https://github.com/etcd-io/etcd/issues/12652

We propose here 3 packages:
 - clientv3/naming/endpoints ->
    That is abstraction layer over etcd that allows to write, read &
    watch Endpoints information. It's independent from GRPC API. It hides
    the storage details.

 - clientv3/naming/endpoints/internal ->
    That contains the grpc's compatible Update class to preserve the
    internal JSON mashalling format.

 - clientv3/naming/resolver ->
   That implements the GRPC resolver API, such that etcd can be
   used for connection.Dial in grpc.

Please see the grpc_naming.md document changes & grpcproxy/cluster.go
new integration, to see how the new abstractions work.

Signed-off-by: Chao Chen <chaochn@amazon.com>

etcdserver/api/rafthttp/transport.go

@@ -114,7 +114,7 @@ type Transport struct {
 	Raft        Raft       // raft state machine, to which the Transport forwards received messages and reports status
 	Snapshotter *snap.Snapshotter
 	ServerStats *stats.ServerStats // used to record general transportation statistics
-	// used to record transportation statistics with followers when
+	// LeaderStats is used to record transportation statistics with followers when
 	// performing as leader in raft protocol
 	LeaderStats *stats.LeaderStats
 	// ErrorC is used to report detected critical errors, e.g.,