Move pairing agent to bluetooth-daemon

[danirabbit]

Problem

Pairing agent is only available while system settings is running, It should be available all the time since bluetooth pairing can be initiated from other places than system settings

Proposal

Move /switchboard-plug-bluetooth/blob/main/src/Services/Agent.vala and associated classes to https://github.com/elementary/bluetooth-daemon

Prior Art (Optional)

No response

[danirabbit]

Gonna close this since discovery is turned off when system settings is closed and we actually shouldn't have pairing in other places