prepare osmo-mgw architecture for other endpoint types
there is still a lot of legacy that ties osmo-mgw to the old concept of "a statically allocated array of rtp-bridge endpoints"
- get rid of endpoint integers and replace it with string (e.g. in log messages)
- dispatch messages like CRCX/MDCX to endpoint class specific operations after message decodin
- implement endpoint name matching based on endpoint class prefix like 'rtpbridge/'
- implement a simple "audio file playback" endpoint endpoint class to verify abstraction is complete