abis: add optional WebSDR E1 input driver
Add a new optional E1 input backend for WebSDR, enabled via
--with-websdr.
Introduce input/websdr.c and public callback API to replace
socket-based IPA transport with a callback-driven interface
for OML/RSL and OSMUX.
Intended for WebAssembly builds where Osmocom components run
in a browser and communicate with SDR via WebUSB.
Change-Id: Ib804dc8eb67a91678603f54ea0beccd5c61e4a80
