drivers: modem: ublox-sara-r4: automatic setting of APN

During communication initialisation, the IMSI of the inserted SIM
card is evaluated to determine the APN. This is done by comparing
the first 5 characters of the IMSI to a list of known providers.
The list can be given in Kconfig.

To enable this functionality, set following bool in Kconfig:
MODEM_UBLOX_SARA_AUTODETECT_APN

To set a list of providers, set following string:
MODEM_UBLOX_SARA_AUTODETECT_APN_PROFILES

If the provider can not be found in the list, the APN given in
following entry is used as a fallback:
MODEM_UBLOX_SARA_R4_APN

Signed-off-by: Hans Wilmers <hans@wilmers.no>