feat(remoteaccess): support remote access connections to devices

[reubenmiller]

Add support for creating remote access configurations and connecting to passthrough connections via a local proxy.

This feature will replace c8ylp as it provides the proxy functionality with zero dependencies on python! The local proxy is actually very simple in golang (compared to python), and allows for a nicer integration with go-c8y-cli.

The local proxy part was added to go-c8y (but still in a feature branch), and the go-c8y-cli is just about getting input from the user and then using the local proxy.

The following functionality is implemented:

• c8y remoteaccess connect ssh (stdio, TCP, socket all implemented)
• c8y remoteaccess server
  • open browser to server address
• Validate long ssh sessions (e.g. > 8 hours)
• c8y remoteaccess configurations subcommands to manage existing connections
• pipeline support (this requires another in-progress feature)
• configuration completion
• allow users to set a default ssh user
• allow users to launch an additional command after starting the server and give the process access to the port number

Below shows an example of an ssh created with the new local proxy commands (without using c8ylp!)

% c8y remoteaccess connect ssh --device rpix-abcdef --user root
Starting interactive ssh session with 991210 (https://example.c8y.io)

Warning: Permanently added '[127.0.0.1]:63005' (ED25519) to the list of known hosts.
Linux rpix-abcdef 6.1.0-rpi7-rpi-2712 #1 SMP PREEMPT Debian 1:6.1.63-1+rpt1 (2023-11-24) aarch64

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
Last login: Tue May 14 21:26:43 2024 from 127.0.0.1
root@rpix-abcdef ~ # pwd
/root
root@rpix-abcdef ~ # exit
Connection to 127.0.0.1 closed.
2024/05/14 22:35:00 Connection closed: D: 127.0.0.1:63005, S: 192.168.68.58:63007
Duration: 9.753s