Provide a way to use Proxy structs instead of `ObjectPath` for in and out arguments of methods & signals

xdg-portals uses a Request interface that has a Response signal that's common between all the portals that might require an interaction by the user. The result of a dbus call like https://flatpak.github.io/xdg-desktop-portal/portal-docs.html#gdbus-method-org-freedesktop-portal-FileChooser.OpenFile will be a Request object path like /org/freedesktop/portal/desktop/request/1_279/t.

What would be neat is to be able to convert the String to a Request object that would implement Proxy, if we can pass in the connection from the parent, it would make chaining thing easier.

    let connection = zbus::Connection::new_session()?;
    let proxy = AccountProxy::new(&connection)?;

    let mut options = std::collections::HashMap::new();
    options.insert("reason", "Factal would like to access to your avatar & user name".into());
    proxy.get_user_information("az", options)?.response(|results| {

    });

Note, the response is a signal and that depends on how those would be implemented in the future.

The current AccountProxy

#[dbus_proxy(
    interface = "org.freedesktop.portal.Account",
    default_service = "org.freedesktop.portal.Desktop",
    default_path = "/org/freedesktop/portal/desktop"
)]
/// The interface lets sandboxed applications query basic information about the user,
/// like his name and avatar photo.
///
/// The portal backend will present the user with a dialog to confirm which (if any) information to share.
trait Account {
    /// Gets information about the user.
    ///
    /// # Arguments
    ///
    /// * `window` - Identifier for the window
    /// * `options` - A HashMap
    ///     * `handle_token` - A string that will be used as the last element of the handle. Must be a valid object path element.
    ///     * `reason` - A string that can be shown in the dialog to expain why the information is needed.
    fn get_user_information(
        &self,
        window: &str,
        options: HashMap<&str, Value>,
    ) -> Result<String>;

    /// version property
    #[dbus_proxy(property)]
    fn version(&self) -> Result<u32>;
}

My idea is to be able to have

    fn get_user_information(
        &self,
        window: &str,
        options: HashMap<&str, Value>,
    ) -> Result<Request>;

While Request being a Struct that would look like this? not sure really how feasible or if logical to have such API

struct Request(String);

impl Request {
    /// Closes the portal request to which this object refers and ends all related user interaction (dialogs, etc).
    /// A Response signal will not be emitted in this case.
    fn close(&self) -> Result<()>;

    // signal
    //fn response(&self) -> Result<(ResponseType, HashMap<&str, Value>)>;
}