Update secret-service to v2.0

This MR upgrades the secret-service dependency to 2.0, which is now using zbus, so more Rust code coverage!

Furthermore, a major architectural change was made: fractal now opens a single connection to the secret service and keeps it alive during the entire life time of the program. This cleans up some of the code and removes the requirement of always having to open an new secret service connection for every secret storage operation.