Changes

add support for RTP extensions via TW-TS-003

Themyscira Wireless Technical Specification TW-TS-003 defines
a mechanism whereby the CN can ask the BSS (via an extension to
BSSMAP Assignment procedure) to use enhanced RTP payload formats
that are contrary to the stipulations of 3GPP TS 48.103.
However, these RTP extensions need to originate at the BTS,
hence if the BSC receives a request from the CN to use them,
it needs to validate that request against BTS capabilities,
and then actually pass it to the BTS via an Osmocom-defined
RSL IE that replicates TW-TS-003 in Abis.  Implement this logic.

Intra-BSC handovers: if ThemWi RTP extensions are used in a GSM
network that allows handovers, all deployed BTSes must have
the same capabilities with regard to these extensions, practically
meaning same or close-enough version of OsmoBTS.  OsmoBSC checks
CN-requested RTP extensions against BTS capabilities on the
initial assignment, but not on subsequent handovers.

Inter-BSC handovers: support for RTP extensions in this deployment
configuration (networks that allow such handovers) remains to be
implemented.

OsmoMGW considerations: if all deployed BTSes are native IP
OsmoBTS, as opposed to E1-based, OsmoMGW does not need to know
anything at all about ThemWi RTP extensions.  However, future
support for these RTP extensions with E1-based BTSes will require
adding a mechanism for communicating the necessary non-standard
request to OsmoMGW.

LCLS considerations: enabling ThemWi RTP extensions affects only
the uplink RTP output path in OsmoBTS and not the path from RTP
input to downlink Tx.  The latter path accepts both basic and
extended RTP formats; any ThemWi extensions in the incoming RTP
stream are harmlessly ignored.  Therefore, activation of ThemWi
RTP extensions will have no impact on LCLS operation.

Related: OS#6448
Depends: I0eccfe5ddcf44f8f20440acb01e2d4870ec0cd91 (libosmocore)
Change-Id: I2a742afff160a9a8286633bf5ae0dd4a828c7a42