[ntcore] Add client disconnect function (#5022)

As setServer doesn't disconnect, it's useful to have a function that
disconnects without needing to completely stop the client.

ntcore/src/generate/java/NetworkTableInstance.java.jinja

@@ -911,6 +911,14 @@ public final class NetworkTableInstance implements AutoCloseable {
     NetworkTablesJNI.setServerTeam(m_handle, team, port);
   }
 
+  /**
+   * Disconnects the client if it's running and connected. This will automatically start
+   * reconnection attempts to the current server list.
+   */
+  public void disconnect() {
+    NetworkTablesJNI.disconnect(m_handle);
+  }
+
   /**
    * Starts requesting server address from Driver Station. This connects to the Driver Station
    * running on localhost to obtain the server IP address, and connects with the default port.

ntcore/src/generate/java/NetworkTablesJNI.java.jinja

@@ -251,6 +251,8 @@ public final class NetworkTablesJNI {
 
   public static native void setServerTeam(int inst, int team, int port);
 
+  public static native void disconnect(int inst);
+
   public static native void startDSClient(int inst, int port);
 
   public static native void stopDSClient(int inst);