Ensure xDS HTTP filters have access to the serialized bytes of each message

This is taken verbatim from the ext_proc gRFC A102: ``` gRPC implementations should structure their xDS HTTP filter APIs such that the filter has access to the serialized bytes of each message rather than the deserialized message, to avoid the overhead of repeated serialization/deserialization on each message. In other words, on the client side, the filters should run after the message is serialized, and on the server side, the filters should run before the message is deserialized. ``` Currently, our xDS HTTP filters can wrap the stream implementation in the gRPC channel to override any of the stream functionality. But the `SendMsg` and `RecvMsg` methods accept protobuf structs and not serialized bytes of the message. The serialization is triggered only from the stream implementation in the gRPC channel: https://github.com/grpc/grpc-go/blob/9a130aad0775eec6d573e1c83a558f9039073b9c/stream.go#L958 This means that we incur additional serialization and deserialization costs for every message sent and received to/from the proc_server.