Changes

mgcp_client: add support for VND.THEMWI.CCSD codec

TW-TS-007 defines a compressed RTP payload format for GSM CSD,
reducing the payload size from 160 bytes of mostly constant filler
bits to 16, 32 or 37 bytes depending on specific CSD mode:

https://www.freecalypso.org/specs/tw-ts-007-v010001.txt

Clause 9 specifies payload type number 127 when this pseudo-codec
is used on AoIP interface, while Annex A gives it the name
VND.THEMWI.CCSD for SDP.

Add support for this codec to libosmo-mgcp-client, to make it
possible for OsmoBSC and OsmoMSC to request this codec from MGWs.

Beyond the present client library addition, desired support for
compressed CSD per TW-TS-007 in Osmocom components is as follows:

* Equip osmo-mgw rtpbridge with ability to convert between CCSD
  and CLEARMODE on the fly, using already merged libosmotrau
  functions for the actual conversion;

* Add CCSD support to tw-e1abis-mgw while regular osmo-mgw has no
  CSD support at all in E1 mode; possibly bring tw-e1abis-mgw into
  Osmocom at some point;

* Have OsmoBSC direct its MGW to emit CCSD when the CN requested it
  via TW-TS-003 BSSMAP extension;

* Possibly add compressed CSD support to OsmoMSC.

Change-Id: I6e4e15b5b3d20036aae8472bd94bc878b718ddd4