net: wifi_mgmt: Support to configure AP parameter max_num_sta

Support to set BSS parameter "max_num_sta" at compile and run time

Added support to configure `max_num_sta` BSS parameter.
Maximum number of stations allowed in station table. New stations will be
rejected after the station table is full.

Signed-off-by: Ajay Parida <ajay.parida@nordicsemi.no>