trx: add JSON serialization for 'show trx'
Introduce trx_to_json() helper to serialize the output of
'show trx' into JSON format.
Also initialize talloc/VTY context in websdr entrypoint to
allow accessing TRX configuration programmatically.
Intended for external consumers (e.g. WebSDR frontend).
Change-Id: Icbfb754580daa6db9b5646cc6698a6a6ee1c1610
