Allow MGCP clients to pass arbitrary fmtp strings to MGWs, beyond the fixed set of parameters captured in legacy struct mgcp_codec_param, and set a different fmtp string per payload type, as opposed to global.
Credit: this patch is a derivative work based on Neels Hofmeyr's patch If58590bda8627519ff07e0b6f43aa47a274f052b from WIP branch neels/sdp, reduced to just libosmo-mgcp-client.
Present necessity: this functional addition is needed in order to allow osmo-bsc to pass this construct to its MGW when the CN requested the use of TW-TS-006 extended payload format for AMR:
AMR codec fmtp parameter tw-ts-006 (defined in TW-TS-006 spec section B.1) is not supported by osmo-mgw; however, it is supported by tw-e1abis-mgw, which is the OsmoBSC-compatible MGW needed for E1 BTS with AMR and CSD.
