Changes

bts_features: add feature flags for TWTS001 and TWTS002

TW-TS-001 and TW-TS-002 (Themyscira Wireless Technical Specifications)
are enhanced RTP payload formats that replicate the functionality and
semantics of 3GPP TS 48.060 and 48.061 (respectively) over IP transport.
Companion spec TW-TS-003 defines a BSSMAP extension whereby the CN
tells the BSS that it wishes to use these otherwise non-standard RTP
formats - but these RTP extensions need to originate at the BTS, as
their main purpose is to pass along information that is otherwise lost
in RTP transport with standard payload formats.

Define feature flags whereby OsmoBTS can tell OsmoBSC that it supports
these RTP extensions.

Change-Id: Ia2cd1d5fa37e9d10927ca5d6cad79ec538fd0a11