Changes

add vty setting for ThemWi RTP formats

For every GSM speech codec and for CSD, Osmocom BSS supports both
the standard RTP format prescribed by 3GPP and an alternative enhanced
RTP format invented by Themyscira Wireless: TW-TS-001, TW-TS-002,
TW-TS-006 and TW-TS-007.  In order to enable these enhanced RTP
formats, the MSC needs to include an extension IE in the Assignment
Request it sends to the BSS.  Until now there was no way to do so
in mainline OsmoMSC, only via local code patches - fix this omission.

The following considerations apply to this new tw-rtp-formats
vty setting:

* When internal MNCC is used, it is beneficial to enable enhanced TW
  RTP formats, especially when using E1 BTS with tw-e1abis-mgw:
  the DL call leg receives more complete information content from
  the UL call leg in each direction.

* When external MNCC is used with non-ThemWi CN software (e.g., with
  a PBX via osmo-sip-connector), tw-rtp-formats should NOT be
  enabled - off-the-shelf PBX etc software won't understand them.

* Future patches will allow osmo-msc to operate with a ThemWi
  transcoding MGW (TC to G.711) in the place of non-TC osmo-mgw;
  in that configuration TW RTP formats will become mandatory,
  as they are strictly required by ThemWi TC implementation.

Change-Id: Icea152f5f2cdbcb0df09ef5b7211d1461b499f14