Expose a way to serialize/deserialize headers and body for different arg schemes

[abhinav]

Now that we support fallback endpoints, it is possible to implement endpoints that work with multiple arg schemes. It would be nice if we provided some way to take the raw bytes of the header or the body from the request and deserialize them based on whatever arg scheme request is using, and do the reverse for raw bytes of the header/body of the response.

Currently it's possible to use JsonSerializer and ThriftSerializer directly but they're not part of the public API and shouldn't be exposed as-is.