For Digi LTE put in PSM

[neilh10]

Currently when making a call the tower connection takes 26seconds.
There is supposed to be faster time using PSM that maintains the state of the cell tower connection, even while going into low power mode.
For the Digi LTE XB3 CAT-M1 there is a mode PSM - Power Save Mode - that allows a connection to maintained with the cell tower through shutdown. See comments
c7846be
Its supposed to enable XB3-C-A2-UT-001 CAT-M1 to wake up faster (and save power)
The question is will it be worth it for the XB3-C-A2-UT or could it be superseded by XB3-C-GM2-UT-001

This likely requires changes to DigiXBeeCellularTransparent.cpp and to TinyGSM : TinyGsmClientXBee.h

Hopefully the SIM7080G also has the same mode, and it may be the right