Draft: Enable AT command interface in NR UE

This MR is a continuation of !2485.

Changes

1. Changes to NAS to send multiple PDU session request.
2. Enable AT command interface. The command parser and API already exists. Added new handler functions to set and activate PDP contexts.
3. Extended parser function to parse NSSAI from AT command.
4. Added two command line parameters. One to enable AT-command interface (--AT) and another to specify the virtual serial port to be used by the AT interface (--AT-interface).

Usage

1. Run socat -d -d pty,raw,echo=1 pty,raw,echo=1 to connect two virtual serial interfaces. This outputs two interface names. Example output

   2024/05/19 12:53:59 socat[347345] N PTY is /dev/pts/12
   2024/05/19 12:53:59 socat[347345] N PTY is /dev/pts/13

2. Run UE with --AT --AT-interface "/dev/pts/12"
3. Open the second interface with screen or minicom to send AT commands. Ex., minicom -D /dev/pts/13
4. Supported commands: at+cgdcont and at+cgact.
   • at+cgdcont? this displays the configured PDP context.
   • at+cgdcont=1,"IP","oai",,,,,,,,,,,,,,1,"01.16777215", configure the second PDP context. Syntax according to 3GPP TS 27.007 10.1.1.
   • at+cgdcont=2,"IP","oai",,,,,,,,,,,,,,1,"01.16777215", configure the third PDP context.
   • at+cgact=1,1 activate second context.
   • at+cgact=1,2 activate third context.
   • at+cgact? get the status of configured contexts.