rtsp-client: add overridable generic handle_request and handle_data vmethods

Submitted by Hyunjun Ko `@zzoon`

Created attachment 281175
RTSP msgs on miracast

I'm just starting to implement wifi display based on gst-rtsp-server.

There are some rtsp msgs based on wifi display spec, which means that i need to implement new handling logic for these msgs.

Of course I can handle these msgs by creating new method. But I think overriding handle_request in rtsp-client class would be better.

Please find the attached file, miracast rtsp msgs to be more easy to understand.
And also attached patch.

Could you review this?

Attachment 281175, "RTSP msgs on miracast":