ControllerInterface: Add InputBackend interface and SDL implementation.

Source/Core/InputCommon/ControllerInterface/InputBackend.h

@@ -0,0 +1,26 @@
+// Copyright 2022 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+class ControllerInterface;
+
+namespace ciface
+{
+class InputBackend
+{
+public:
+  InputBackend(ControllerInterface* controller_interface);
+
+  virtual ~InputBackend();
+
+  virtual void PopulateDevices() = 0;
+  virtual void UpdateInput();
+
+  ControllerInterface& GetControllerInterface();
+
+private:
+  ControllerInterface& m_controller_interface;
+};
+
+}  // namespace ciface