[mod_opus] add cfg setting to overwrite the fmtp stereo param coming from remote...

[mod_opus] add cfg setting to overwrite the fmtp stereo param coming from remote. Eg: incoming SDP has stereo=1 but we want a mono call so we answer with stereo=0.

* [mod_opus] add cfg setting to overwrite the fmtp stereo param coming from remote. Eg: incoming SDP has stereo=1 but we want a mono call so we answer with stereo=0.
sprop-stereo will be set to 0 too.

* [core] opus: use switch_core_max_audio_channels() with remote fmtp stereo=1 to allow disabling of stereo.