Refactor header validation and add GetSessionState scaffold.

src/Socialbox/Classes/StandardMethods/GetSessionState.php

@@ -0,0 +1,20 @@
+<?php
+
+    namespace Socialbox\Classes\StandardMethods;
+
+    use Socialbox\Abstracts\Method;
+    use Socialbox\Interfaces\SerializableInterface;
+    use Socialbox\Objects\ClientRequest;
+    use Socialbox\Objects\RpcRequest;
+
+    class GetSessionState extends Method
+    {
+
+        /**
+         * @inheritDoc
+         */
+        public static function execute(ClientRequest $request, RpcRequest $rpcRequest): ?SerializableInterface
+        {
+            // TODO: Implement execute() method.
+        }
+    }