[hal] Add high level way of getting systemserver NT instance (#7683)

hal/src/main/native/systemcore/SystemServerInternal.h

@@ -0,0 +1,13 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include <networktables/NetworkTableInstance.h>
+
+namespace hal {
+void InitializeSystemServer();
+void ShutdownSystemServer();
+nt::NetworkTableInstance GetSystemServer();
+}  // namespace hal