[FEATURE] Opt-in automatic secret-agent re-registration after NetworkManager restart

[cachebag]

Is your feature request related to a problem? Please describe.
SecretAgentHandle::reregister()` currently requires each NMRS consumer to detect NetworkManager restarts, wait for the new D-Bus owner to become available, and retry registration as needed.

Describe the solution you'd like
Add an opt-in SecretAgentBuilder recovery policy that:

• watches org.freedesktop.NetworkManager D-Bus owner changes;
• re-registers only after a new owner is present;
• retries failed registration with bounded backoff during NM startup;
• stops recovery when the handle is unregistered or dropped;
• reports recovery failures through an exposed event/error mechanism.

Ref: pop-os/cosmic-applets#1477