Make base-64 encoding of SDP optional

[travishaagen]

format/webrtc3/adapter.go and format/webrtc/adapter.go assume that the SDP message is base-64 encoded.

func (element *Muxer) WriteHeader(streams []av.CodecData, sdp64 string) (string, error) {
	...
	sdpB, err := base64.StdEncoding.DecodeString(sdp64)

However, I do not think this is required in the specifications.

I see that RTSPtoWebRTC/web/static/js/app.js uses this encoding as well. Perhaps the encoding could be isolated to only the RTSPtoWebRTC project?

If it is not required by the relevant specifications, I think this encoding should be optional / configurable, or removed entirely.

[deepch]

This was in the previous PION example. I think this is not necessary at all.