webrtc: Add python sendonly h264 example with data channels for browser client
The python script includes a built in signaling mechanism using websockets. The browser client HTML uses feross/simple-peer in javascript. Please note that sending data works only one way (from python to browser) with gstreamer version 1.16, but should work both ways with 1.17+. This demo serves as PoC for backporting the datachannel fixes in 1.16 branch.