feat: improve CLI completion for lxc auth subcommands

[simondeziel]

Please confirm

• I have searched existing issues to check if an issue already exists for my feature request.

Is your feature request related to a problem? Please describe.

Some of the lxc auth subcommands are missing CLI completion

Describe the solution you'd like

It'd be nice if lxc auth group delete would autocomplete based on existing groups:

root@micro1:~# lxc auth group list
+-------------+-------------+
|    NAME     | DESCRIPTION |
+-------------+-------------+
| e2e-testing |             |
+-------------+-------------+
root@micro1:~# lxc auth group delete <tab>

Similarly, lxc auth group permission add should also propose completing based on existing groups.

Also, lxc auth identity group add should propose completing existing entities and groups:

root@micro1:~# lxc auth identity list
+-----------------------+--------------------+-------------+------------------------------------------------------------------+-------------+
| AUTHENTICATION METHOD |        TYPE        |    NAME     |                            IDENTIFIER                            |   GROUPS    |
+-----------------------+--------------------+-------------+------------------------------------------------------------------+-------------+
| tls                   | Client certificate | e2e-testing | 6c9e84dc87c5e7b6f90d43e7e5055b2121a0e28549c8a84936192e8455e27108 | e2e-testing |
+-----------------------+--------------------+-------------+------------------------------------------------------------------+-------------+
| tls                   | Server certificate | micro1      | 9528207a660b89fa4b82e1b20d415f68019c92e513d6e3a747b707b21c37ead0 |             |
+-----------------------+--------------------+-------------+------------------------------------------------------------------+-------------+
| tls                   | Server certificate | micro2      | a1d5ef70354a730d4fff99fe9260ebb091e810cf58e3fcecf78ed2eeb941c33b |             |
+-----------------------+--------------------+-------------+------------------------------------------------------------------+-------------+
| tls                   | Server certificate | micro3      | 415c0ff0bb007bc4b5ddd249a956b3fa342283aeccdac2b25cf19284caab74be |             |
+-----------------------+--------------------+-------------+------------------------------------------------------------------+-------------+
root@micro1:~# lxc auth group list
+-------------+-------------+
|    NAME     | DESCRIPTION |
+-------------+-------------+
| e2e-testing |             |
+-------------+-------------+

# this is what I would have liked to tab complete to:
root@micro1:~# lxc auth identity group add tls/e2e-testing e2e-testing

Describe alternatives you've considered

No response

Additional context

No response

[markylaing]

I'm on it :)