Add a way to connect to an already running instance of mavsdk server

Sources/Mavsdk/Drone.swift

@@ -35,6 +35,9 @@ public class Drone {
         case connectionStopped
     }
 
+    /**
+     * Create an instance of MavsdkServer and running with `systemAddress`, which is the address of the drone.
+     */
     public func connect(systemAddress: String = "udp://:14540") -> Completable {
         return Completable.create { completable in
             self.mavsdkServer = MavsdkServer()
@@ -57,6 +60,17 @@ public class Drone {
         }
     }
 
+    /**
+     * Connect MAVSDK to an already-running instance of MavsdkServer, locally or somewhere on the network.
+     */
+    public func connect(mavsdkServerAddress: String, mavsdkServerPort: Int32 = 50051) -> Completable {
+        return Completable.create { completable in
+            self.initPlugins(address: mavsdkServerAddress, port: mavsdkServerPort)
+            completable(.completed)
+            return Disposables.create()
+        }
+    }
+
     private func initPlugins(address: String = "localhost", port: Int32 = 50051) {
         self.action = Action(address: address, port: port, scheduler: scheduler)
         self.calibration = Calibration(address: address, port: port, scheduler: scheduler)