drivers: modem: Add support for quectel bg95

Adding support for Quectel BG95 Modem offloaded driver
to zephyr.

The driver currently implements only the
client side functions of the "socket_op_vtable", and
so cannot be used for cases where Zephyr acts as a
server. Moreover the driver only supports TCP for now.

Looking through the guides, the same driver should be
usable for BG96 (and other modems) except for the modem
boot-up sequence. Hence its named as "bg9x" instead of
"bg95".

Tested extensively with Zephyr acting as MQTT endpoint
and publishing / subscribing data to / from an MQTT
broker.

Signed-off-by: Bilal Wasim <bilalwasim676@gmail.com>