osmo-bts: 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.
So we don't need to support IMMEDIATE ASSIGNMENT EXTENDED in osmo-bsc, we can already craft them in osmo-bts (bts.c) in a way similar to what we already do with IMMEDIATE ASSIGNMENT REJECT commands (we couple up to 4 messages in one in there).
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.