device: add WebSDR radio backend
Introduce a new optional WebSDR device backend, enabled via
--with-websdr.
Add WebSDRDevice implementation and build integration, providing
a RadioDevice interface backed by callback hooks for control and
sample I/O.
Intended for Web-based deployments where osmo-trx interacts with
SDR hardware via a WebSDR/WebUSB frontend.
Change-Id: Ie459cbd70388dd8ff5b89221d30770bab0bd9014
