[Feature request] Display repository GPG key (signed-by) in Software Sources

[SanAndreyas]

Resolves #268

Since APT now recommends per-repository GPG keys using the signed-by= directive (stored in /usr/share/keyrings/), the Software Sources tool no longer shows any authentication keys, even though they are correctly configured and used by APT.

This can be confusing for users, especially those who added third-party repositories manually and want to audit or verify their system configuration.

Suggested improvement
For each enabled repository, display (read-only):

GPG key file used (signed-by=…)
Key fingerprint (optional)
Key location (/usr/share/keyrings/…)
Status (key present / missing)
This information could be shown in an “Advanced” or “Details” view to keep the UI simple for non-technical users.

Benefits

Improves transparency and trust in third-party repositories
Aligns the GUI with modern APT security practices
Helps users audit and reproduce clean system installations
Reduces confusion caused by the now-empty “Authentication keys” section