Add E1/T1 endpoint type to osmo-mgw
Contrary to OsmoNITB, OsmoBSC has no user plane / voice handling inside it. Rather, all media/voice handling is done inside osmo-mgw.
We will not be able to support classic E1/T1 BTSs from OsmoBSC unless we're able to support E1/T1 endpoints in osmo-mgw. So let's implement them. We'll at minimum need endpoint types for 16kbit sub-slots with TRAU frames (voice calls).
TBD is the details on how the signaling is handled. We could possibly open signaling slots directly from the BSC (via libosmo-abis/e1_input), or we could have osmo-mgw open those slots and convert the HDLC payload / LAPD into something that can be transported over IP to osmo-bsc. This is related but a separate ticket.