Expose Encodings variable for ORTC

[kixelated]

It's not possible to use a RTPReceiver without it.

[kixelated]

I've been trying to figure out if the SSRC is actually required and I'm not sure. There doesn't seem to be a convenient way to get this information from the API like some of the other information required for signaling. Do you even need to signal the track SSRC?

c.receiver.Receive(webrtc.RTPReceiveParameters{
  Encodings: webrtc.RTPDecodingParameters{
    webrtc.RTPCodingParameters{SSRC: ssrc},
  },
})

[coveralls]

Pull Request Test Coverage Report for Build 2216

• 6 of 6 (100.0%) changed or added relevant lines in 3 files are covered.
• 13 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.2%) to 79.913%

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Files with Coverage Reduction	New Missed Lines	%
dtlstransport.go	4	75.71%
peerconnection.go	9	74.74%

Totals
Change from base Build 2214:	-0.2%
Covered Lines:	3859
Relevant Lines:	4829

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💛 - Coveralls