Simulcast and per ssrc-encryption

[youennf]

As pointed out offline by @k-wasniowski, in case of simulcast, there is currently no way to specify per-ssrc encryption for SFrameTransform.
This is implementable with script transform.
Either we are ok with the limitation to use one encryption key for all simulcast streams, or we need to beef up the API, for instance by having setEncryptionKey take an optional rid parameter.

[k-wasniowski]

Some of the WebRTC users are still using legacy(ssrc-based) simulcast. Would this API take it into consideration?
@youennf