Adapt storage proposal to the new architecture (#1175)

This PR implements an HTTP/JSON API for managing the storage proposal
and all the support methods needed by the corresponding web interface.

This works as a proxy over the real API which is implemented in D-Bus.
That comes with some problems (especially regarding performance) that
will need to be fixed in the future.

## Testing

- Existing unit tests adapted to the new API, except those related to
signals (`onDeprecate` and `onIssuesChange`).
- Tested manually

## References

https://trello.com/c/HOkBiVHp/

rust/agama-lib/src/dbus.rs

@@ -1,9 +1,6 @@
-use anyhow::Context;
 use std::collections::HashMap;
 use zbus::zvariant::{self, OwnedValue, Value};
 
-use crate::error::ServiceError;
-
 /// Nested hash to send to D-Bus.
 pub type NestedHash<'a> = HashMap<&'a str, HashMap<&'a str, zvariant::Value<'a>>>;
 /// Nested hash as it comes from D-Bus.

rust/agama-lib/src/storage.rs

@@ -1,8 +1,8 @@
 //! Implements support for handling the storage settings
 
 pub mod client;
-pub mod device;
-mod proxies;
+pub mod model;
+pub mod proxies;
 mod settings;
 mod store;