Improve D-Bus network interface (#961)

## Problem

Until now, the communication between our network model and its D-Bus
interface worked only in one direction (from D-Bus to the model). And
that was pretty limiting.

## Solution

In #885 we introduce the usage of `oneshot::channel()` to enable
bi-directional communication between the model and the D-Bus interface.
It worked rather well, so it is time to extend this approach.

Some use cases that are covered:

* Perform a complex validation in the model (considering the whole
model) and return the result to the D-Bus interface.
* Return the D-Bus path of a new connection (implemented in this PR).

But there is another interesting implication: the model is the single
source of truth (no more cloned data in the interface). In the future,
we are expected to listen to NetworkManager and update some parts of our
model accordingly. Not having to replicate that information to D-Bus
(except when new things add or disappear) makes things easier.

Note: `org.opensuse.Agama1.Network.Device` and
`org.opensuse.Agama1.Network.Devices` are excluded by now.

## Testing

- *Added a new unit test*
- *Tested manually*


## To do

- [x] Reduce the boilerplate associated to the usage of channels.

rust/agama-dbus-server/Cargo.toml

@@ -25,3 +25,4 @@ gettext-rs = { version = "0.7.0", features = ["gettext-system"] }
 regex = "1.10.2"
 once_cell = "1.18.0"
 macaddr = "1.0"
+async-trait = "0.1.75"

rust/agama-dbus-server/src/network/action.rs

@@ -2,6 +2,7 @@ use crate::network::model::Connection;
 use agama_lib::network::types::DeviceType;
 use tokio::sync::oneshot;
 use uuid::Uuid;
+use zbus::zvariant::OwnedObjectPath;
 
 use super::error::NetworkStateError;
 
@@ -15,7 +16,11 @@ pub type ControllerConnection = (Connection, Vec<String>);
 #[derive(Debug)]
 pub enum Action {
     /// Add a new connection with the given name and type.
-    AddConnection(String, DeviceType),
+    AddConnection(
+        String,
+        DeviceType,
+        Responder<Result<OwnedObjectPath, NetworkStateError>>,
+    ),
     /// Gets a connection
     GetConnection(Uuid, Responder<Option<Connection>>),
     /// Gets a controller connection
@@ -25,9 +30,13 @@ pub enum Action {
     ),
     /// Sets a controller's ports. It uses the Uuid of the controller and the IDs or interface names
     /// of the ports.
-    SetPorts(Uuid, Vec<String>, Responder<Result<(), NetworkStateError>>),
+    SetPorts(
+        Uuid,
+        Box<Vec<String>>,
+        Responder<Result<(), NetworkStateError>>,
+    ),
     /// Update a connection (replacing the old one).
-    UpdateConnection(Connection),
+    UpdateConnection(Box<Connection>),
     /// Remove the connection with the given Uuid.
     RemoveConnection(String),
     /// Apply the current configuration.