Changes

Introduce struct osmo_ss7_as_asp_assoc

This new struct allows supporting unlimited number of ASPs per AS.
It can also potentially be used in the future to:
* Hold M3UA "ASP Identifier" per AS<->ASP association
* Speed up iterating over AS served by a given ASP.

NOTE: Having >16 ASPs per AS may not be desirable, for instance under
loadshare traffic mode ITU SLS is 4 bit, so despite having ext_sls
extending the table to 128 entries based on OPC, it may not provide
enough entropy to properly loadshare among all of them.

Related: OS#7519
Change-Id: I149ab467899633ac50cba3e482b2cae02124279d