trx: add JSON stats export for WebSDR API
Introduce stats_json.c providing JSON serialization for:
- stats and counters
- rate counters
- BTS state
- TRX/transceiver state
- WebSDR runtime metrics
Add osmobts_get_stats() API to expose these via WebSDR
interface and export it in Emscripten build
This enables programmatic access to data previously available
via VTY (e.g. "show stats", "show bts")
Change-Id: Ifb82adfab879a65ecd222c45e06551983aa90a0f
