audio/rsl: Include statistics for one call
audio/rsl: Include statistics for one call

Use the new libosmo-abis API to query the session for the
statistics and then send it as a TLV element to the BSC.
This can be used to do post processing about the call. E.g
to figure out if no audio arrived at all.

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts