drivers: modem: add at shell

Add AT shell which uses the newly exposed pipes to
send/receive at commands from the modem identified by the
chosen node zephyr,at-shell.

To send an AT command, the shell command at is used, followed
by the command to send, and an optional response which
overwrites the default "OK"

For example, sending "AT", which returns "OK"
  modem at at <- command
  OK <- response

Enabling echo, then sending "AT" which will now return "AT" + "OK"
  modem at ati1 <- command
  OK <- response
  at at <- command
  at <- response
  OK <- response

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>