netutils/dropbear: add scp support via SSH exec requests

[FelipeMdeO]

Summary

The initial Dropbear port (#) only supports interactive
shell sessions;
This change adds SCP support to the Dropbear server port.

This PR is work continuation of: #3561

Impact

• No impact when CONFIG_NETUTILS_DROPBEAR_SCP is disabled.
• When enabled, a new scp builtin is registered and the Dropbear server
  accepts SSH exec requests, which are executed as NSH commands.

Testing

Target: ESP32-C3 (esp32c3-devkit:dropbear, with CONFIG_PIPES=y and
CONFIG_NETUTILS_DROPBEAR_SCP=y).

Copying a file from the host to the board:

$ echo hello > pc.txt
$ scp -O -P 2222 pc.txt root@<board-ip>:/data/pc.txt
    root@<board-ip>'s password:
    pc.txt                                    100%    6     0.1KB/s   00:00

Board side log:

    nsh> ifconfig
    wlan0   Link encap:Ethernet HWaddr XX:XX:XX:XX:XX:XX at RUNNING mtu 1500
            inet addr:<board-ip> DRaddr:<gateway-ip> Mask:255.255.255.0

    nsh> [7] Jan 01 00:01:24 connection from <host-ip>:34666
    [7] Jan 01 00:01:30 Password auth succeeded for 'root' from <host-ip>:34666
    [7] Jan 01 00:01:30 NSH exec started: scp -t /data/pc.txt
    [8] Jan 01 00:01:30 NSH session exited: 0
    [7] Jan 01 00:01:30 Exit (root) from <<host-ip>:34666>: Disconnect received
    nsh> ls /data
    /data:
     dropbear_ecdsa_host_key
     passwd
     pc.txt

    nsh> cat /data/pc.txt
    hello