Allow createEncodedStreams on PCs without encodedInsertableStreams parameter

[kjvenalainen]

Fixes #299

Allows calling clients to add sender and receiver callback functions which are executed synchronously after RTCRTPSender/Receiver creation. This feature is targeted to Chromium browsers and allows applications to call createEncodedStreams() in the callback functions in order to create WebRTC Encoded Transforms.

Example Usage

const consumer = await recvTransport.consume({
  xxxxx,
  xxxxx,
  onRtpReceiver: (receiver) => {
    const { readable, writable } = receiver.createEncodedStreams();
    readable.pipeTo(writable);
  }
});

[ibc]

** please

[ibc]

Same

[ibc]

I tried to push to this repo following GH instructions and... ended creating another PR #301 by accident (not sure what I did wrong). Let's continue in there. Sorry.