Allow setting RtpEncodingParameters for individual encodings

[t-mullen]

Description

Changes Producer::setRtpEncodingParameters to also accept an array of RTCRtpEncodingParameters, for setting different parameters for each encoding. Passing a single RTCRtpEncodingParameters object will work as before.

Usecases

I have a few uses for this - just some examples:

1. Disabling lower-indexed encodings (similar to setMaxSpatialLayer but reversed).
2. Modifying scaleResolutionDownBy depending on the current source resolution of the track.

[ibc]

This is wrong. Old matching properties override the new ones.

[t-mullen]

This is the way property assignment was done before. It seems to be correct:

const oldObj = { x: 1 }
const newObj = { x: 2 }
{ ...oldObj, ...newObj } // { x: 2 }

It transpiles to:

parameters.encodings[idx] = Object.assign(Object.assign({}, encoding), params[idx]);

Which also seems right.

[ibc]

I'm in mobile and could be wrong. I'll review in next days. Thanks

[jmillan]

LGTM

[ibc]

I want to re-review this, sorry for the delay, off for many weeks.