Enable AT command interface and link to NAS layer

1. Enable AT command interface. The command parser and API already exists.
2. Function to create PTY pair to get AT commands.
3. Trigger deregistration with at+cfun=0.
4. Optional command line parameter to user device from user.
5. Added new handler functions to set PDP contexts.
6. Extended parser function to parse NSSAI from AT command. (support for establishing multiple PDU session will be done in following MR).
7. Added documentation for AT command in RUNMODEM.md.

Triggering 4G test cases to make sure I didn't break anything there.

Previous discussion is in !2643 (closed).