fix: usr: Check "asnum" validity in GeoIP ACLs

fix: usr: Check "asnum" validity in GeoIP ACLs

We now check the validity of autonomous system (AS) numbers when
parsing GeoIP ACLs that use `asnum` elements at configuration time.

`asnum` values start with an optional case-insensitive "AS" prefix, followed only by decimal digits, with no spaces or other extraneous characters. The value represented cannot exceed 2^32.

Closes #6255

Merge branch '6255-check-asnum' into 'main'

See merge request isc-projects/bind9!12438