Add support for transcoding to L16 codec on backchannel

[oNaiPs]

Changes:

• Receiver's usage of WriteRTP is wrapped with a Handler function (similar to the Sender struct)
• Add code on webrtc producer to support transcoding from PCMA to PCM (based on https://github.com/AlexxIT/go2rtc/blob/master/pkg/webrtc/consumer.go#L67)

TODO:

• Add resampling functions (@AlexxIT do you happen to have them already?)

Closes #696

[AlexxIT]

I don't think it's a good idea to change the receiver. In all other cases receiver produce codec as is.
There should be a pipeline between the receiver and the sender that converts the codec.