vtprotocodec: user codecV2 for grpc client/server

Switch to use codecV2 backed by the default buffer pool to handle all
Marshaling/Unmarshaling by grpc clients/servers. The default buffer pool
is already setup here.

```
server/util/grpc_client/grpc_client.go
254:		experimental.WithBufferPool(mem.DefaultBufferPool()),

server/util/grpc_server/grpc_server.go
240:		experimental.BufferPool(mem.DefaultBufferPool()),
```

This does not affect the manual Marshaling/Unmarshaling that we does via
`server/util/proto.{Marshal,Unmarshal}()`. These should still use V1 for
the time being.

Author: sluongng

server/util/vtprotocodec/vtprotocodec.go

@@ -1,8 +1,6 @@
 package vtprotocodec
 
 import (
-	"fmt"
-
 	"github.com/buildbuddy-io/buildbuddy/server/util/proto"
 
 	"google.golang.org/grpc/encoding"
@@ -13,30 +11,10 @@ import (
 
 const Name = "proto"
 
-// vtprotoCodec represents a codec able to encode and decode vt enabled
-// proto messages.
-type vtprotoCodec struct{}
-
-func (vtprotoCodec) Marshal(v any) ([]byte, error) {
-	vv, ok := v.(proto.Message)
-	if !ok {
-		return nil, fmt.Errorf("failed to marshal, message is %T, want proto.Message", v)
-	}
-	return proto.Marshal(vv)
-}
-
-func (vtprotoCodec) Unmarshal(data []byte, v any) error {
-	vv, ok := v.(proto.Message)
-	if !ok {
-		return fmt.Errorf("failed to unmarshal, message is %T, want proto.Message", v)
-	}
-	return proto.Unmarshal(data, vv)
-}
-
-func (vtprotoCodec) Name() string {
-	return Name
-}
-
+// vtprotoCodecV2 implements encoding.CodecV2 and uses vtproto and default buffer pool
+// to encode/decode proto messages. The implementation is heavily inspired by
+// https://github.com/planetscale/vtprotobuf/pull/138
+// and https://github.com/vitessio/vitess/pull/16790.
 type vtprotoCodecV2 struct {
 	fallback encoding.CodecV2
 }
@@ -80,7 +58,6 @@ func (c *vtprotoCodecV2) Unmarshal(data mem.BufferSlice, v any) error {
 // RegisterCodec registers the vtprotoCodec to encode/decode proto messages with
 // all gRPC clients and servers.
 func Register() {
-	// encoding.RegisterCodec(vtprotoCodec{})
 	encoding.RegisterCodecV2(&vtprotoCodecV2{
 		fallback: encoding.GetCodecV2("proto"),
 	})