Changes

MGCP extension: add parameters for TW-TS-001 & TW-TS-002

TW-TS-001 and TW-TS-002 are enhanced RTP transport formats for
GSM FR/HR/EFR codecs that restore the lost semantics of GSM 08.60
and 08.61 TRAU-UL frames.

TW-TS-003 BSSMAP extension provides a way for a core network to
ask the BSS to use these TRAU-like enhanced RTP formats instead of
those specified in 3GPP TS 48.103; OsmoBSC already supports this
mechanism when the BSS is comprised of IP-native OsmoBTS.

However, in order to achieve the same effect when using E1-based
legacy BTS hardware, the task of generating TW-TS-001/002 RTP
packets for UL and accepting them for DL moves from OsmoBTS to
the E1-Abis-interfacing MGW.  osmo_trau2rtp() is already capable
of generating these extended RTP formats on request, but until now
there was no mechanism to signal from OsmoBSC to its associated
E1 Abis MGW whether and when these extensions should be used.

Considering that MGCP as it is used in Osmocom is essentially a
private interface between OsmoBSC or OsmoMSC masters and OsmoMGW
slaves, while the externally defined and generally interoperable
interface is 3GPP TS 48.008, possibly extended with TW-TS-003,
the sensible solution is to make a private extension to the way
FR, EFR and HR codecs are described in SDP in the context of
Osmocom-MGCP.

The SDP extension birthed in the present patch consists of an fmtp
parameter for GSM, GSM-EFR and GSM-HR-08 codecs that is structured
just like the already implemented octet-align parameter for AMR.
TW-TS-001 for FR and EFR shall be described as follows:

m=audio 1234 RTP/AVP 3 110
a=rtpmap:3 GSM/8000/1
a=fmtp:3 tw-ts-001=1
a=rtpmap:110 GSM-EFR/8000/1
a=fmtp:110 tw-ts-001=1

TW-TS-002 for HR codec shall be described as follows:

m=audio 1234 RTP/AVP 111
a=rtpmap:111 GSM-HR-08/8000/1
a=fmtp:111 tw-ts-002=1

The present patch adds the new parameters to struct mgcp_codec_param
in mgcp_common.h and equips libosmo-mgcp-client with the ability to
emit these parameters upon request - but no MGW-side implementation
yet.  On the MGW side, this MGCP extension will be implemented at first
in tw-e1abis-mgw, an experimental alternative to OsmoMGW that is
strictly limited to the role of E1-Abis-interfacing MGW working with
OsmoBSC, while implementation in mainline OsmoMGW remains an open
question, to be addressed later based on the outcome of tw-e1abis-mgw
experiments.

Related: OS#6614
Change-Id: I0d58e6d84418f50670c8ab7cf8490af3bc2f5c26