common: add support for TW-TS-001

Themyscira Wireless Technical Specification TW-TS-001 defines
an enhanced RTP transport format for FR and EFR codecs within
an IP-based GSM RAN, restoring the full functionality and semantics
of GSM 08.60 TRAU-UL format that were lost in the industry transition
to RTP with payload formats standardized by TIPHON and IETF.

Given that this new enhanced RTP transport format runs counter
to commonly accepted standards, it is strictly optional.  OsmoBTS
always accepts both basic and extended RTP formats, but it sends
the extended RTP format of TW-TS-001 only when commanded to do so
by the BSC via an RSL extension IE; OsmoBSC will in turn direct
the BTS to use this extension only when the CN asks for it via
the BSSMAP extension defined in TW-TS-003.

Spec references:

https://www.freecalypso.org/specs/tw-ts-001-v010100.txt
https://www.freecalypso.org/specs/tw-ts-003-v010002.txt

Related: OS#6448
Depends: I0eccfe5ddcf44f8f20440acb01e2d4870ec0cd91 (libosmocore)
Change-Id: Id997e8666bc19e60936aaa83b43a968d30320bd7

include/osmo-bts/lchan.h

@@ -168,6 +168,7 @@ struct gsm_lchan {
 		uint16_t conn_id;
 		uint8_t rtp_payload;
 		uint8_t rtp_payload2;
+		uint8_t rtp_extensions;
 		uint8_t speech_mode;
 		struct {
 			bool use;