Add per packet sframe option

[youennf]

Fixes #292

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[k-wasniowski]

@youennf Will ever useSFrame be undefined? Following the steps from the RTCRtpTransceiver initialization, it's always initialized to true/false from the sdp.

Otherwise, [=queue a task=] to run the following steps:
    1. If [=this=].`[[useSFrame]]` is undefined, set [=this=].`[[useSFrame]]` to false.

Which makes me think that we will actually always set it to false?

[youennf]

The first commit of this PR should be reviewed separately at #252. This PR is really only about the second commit.
The principle is the following:

1. If a transceiver is created via addTransceiver or addTrack, useSFrame is initially undefined so that the web app can set the transform in the current event loop task.
2. If a transceiver is created by applying a remote description, useSFrame is initialised according the description.

[k-wasniowski]

@youennf
Should there be a requirement that for sender packet transformers should be applied after frame transformers?
In theory you could create RTCRtpScriptTransform with couple transformers attached like:

onrtctransform = transformer => {
  const encrypter = new RTCSFrameSenderTransform({mode: `packet`);
  transformer.readable
.pipeThrough(new TransformStream(myMetadataTransform1))
    .pipeThrough(encrypter)
    .pipeThrough(new TransformStream(myMetadataTransform2))
    .pipeTo(writable);
};

Where myMetadataTransformX should work on the frame level transformations

[youennf]

RTCSFrameSenderTransform can only be used on RTCRtpSender, not as part of regular whatwg streams.