input: Implement a SDL2 keyboard backend (#2277)

* input: Implement a SDL2 keyboard backend

Add a new keyboard backend to the Ryujinx.Input.SDL2 project.

This is currently unused.

* Address Ac_k's comments

* Address gdkchan's comments

Ryujinx.Input.SDL2/SDLKeyboardDriver.cs

@@ -0,0 +1,54 @@
+using Ryujinx.SDL2.Common;
+using System;
+
+namespace Ryujinx.Input.SDL2
+{
+    public class SDL2KeyboardDriver : IGamepadDriver
+    {
+        public SDL2KeyboardDriver()
+        {
+            SDL2Driver.Instance.Initialize();
+        }
+
+        public string DriverName => "SDL2";
+
+        private static readonly string[] _keyboardIdentifers = new string[1] { "0" };
+
+        public ReadOnlySpan<string> GamepadsIds => _keyboardIdentifers;
+
+        public event Action<string> OnGamepadConnected
+        {
+            add { }
+            remove { }
+        }
+
+        public event Action<string> OnGamepadDisconnected
+        {
+            add { }
+            remove { }
+        }
+
+        protected virtual void Dispose(bool disposing)
+        {
+            if (disposing)
+            {
+                SDL2Driver.Instance.Dispose();
+            }
+        }
+
+        public void Dispose()
+        {
+            Dispose(true);
+        }
+
+        public IGamepad GetGamepad(string id)
+        {
+            if (!_keyboardIdentifers[0].Equals(id))
+            {
+                return null;
+            }
+
+            return new SDL2Keyboard(this, _keyboardIdentifers[0], "All keyboards");
+        }
+    }
+}