modem_cellular: Add support for the Telit ME310G1 LTE modem

Add support for the Telit ME310G1 LTE modem which is very similar to
the ME910G1. Re-use the ME910G1's AT command scripts but rename
them to mex10_xx and define a shutdown script (AT#SHDN).

Signed-off-by: Niklas Gürtler <niklas.guertler@e-obs.de>