async_client: Add a method to fetch the homeserver.

src/async_client.rs

@@ -160,10 +160,16 @@ impl AsyncClient {
         })
     }
 
+    /// Is the client logged in.
     pub fn logged_in(&self) -> bool {
         self.base_client.read().unwrap().logged_in()
     }
 
+    /// The Homeserver of the client.
+    pub fn homeserver(&self) -> &Url {
+        &self.homeserver
+    }
+
     pub fn add_event_callback<C: 'static>(
         &mut self,
         mut callback: impl FnMut(Arc<RwLock<Room>>, Arc<EventResult<RoomEvent>>) -> C + 'static + Send,