Refactor the NetworkSystem API

[imobachgs]

The problem

While working in introducing an API to get the runtime configuration of the network, we decided that we were not happy with the current NetworkSystem API. There was quite some manual to do when using the API, like dealing with channels and having to explictly move the server to a separate Tokio task.

The solution

This change splits the API into smaller pieces:

• NetworkSystem is still the main entry point.
• NetworkSystemClient wraps the actions channel and offers a higher-level API by hiding the message passing handling.
• NetworkSystemServer keeps the network state and dispatches the actions. Beware that this struct is private and the NetworkSystemClient should be the only way to communicate with it.

You can get a better idea of how it looks like through the following example (included in the documentation):

let adapter = NetworkManagerAdapter::from_system()
    .await
    .expect("Could not connect to NetworkManager.");
let dbus = connection()
    .await
    .expect("Could not connect to Agama's D-Bus server.");
let network = NetworkSystem::new(dbus, adapter);

// Start the networking service and get the client for communication.
let client = network.start()
    .await
    .expect("Could not start the networking configuration system.");

// Perform some action, like getting the list of devices.
let devices = client.get_devices().await.unwrap();

To do

• Get rid of NetworkService as it is not needed anymore. Should we rename NetworkSystem*?.
• Adapt the HTTP/JSON interface to the new API (partially done).

[teclator]

LGTM: about renaming.. for me makes sense

[teclator]

LGTM