osmo-bsc: Implement IMMEDIATE ASSIGNMENT EXTENDED
See 04.08 "9.1.19 Immediate assignment extended". It allows to send two Immediate Assignment to different MS in the same message.
Furthermore, according to 08.58 "5.7 IMMEDIATE ASSIGNMENT" section:
The IMMEDIATE ASSIGNMENT EXTENDED message is either sent by the BSC in the IMMEDIATE ASSIGN COMMAND, or built by the BTS from up to two IMMEDIATE ASSIGN COMMAND messages.
It's probably more useful to implement it in osmo-bts (see #3329), but we may still want to support it in osmo-bsc if we connect to non osmo-bts which don't craft IMMEDIATE ASSIGNMENT EXTENDED from regular IMMEDIATE ASSIGNMENT on their own. I imagine for instance an scenario osmo-bsc<->nanobts.
We recently saw some production scenarios which high load in which the AGCH IMM Assign queue was saturated (1000 messages in the queue), so this should help maintain the queue len in lower numbers.