[RFE] Implement DBus communication for client-server architecture

[dongsupark]

Currently ue-rs is a single command-line tool to fetch sysext OEM images, verify signatures, etc. That is sufficient for status of proof-of-concepts for quick implementation.

Ideally that should be split into separate server and client, where they communicate via DBus with each other. That is already done by the production update_engine.

What the server part basically should do:

• Set up a DBus service upon initialization.
  • Get a bus as well as a DBus proxy.
  • Request a DBus name.
  • Register a DBus object.
• Run a scheduler for running periodic update checks.
• Run into a main loop listening for requests.
• Clean up the DBus service upon exit.

What the client part basically should do:

• Get a bus as well as a DBus proxy.
• Send a DBus request AttemptUpdate to the server to trigger update activities.

Rust-native DBus communication could be implemented by using crates like zbus. See also a reference implementation busd.