[Slim] Improve codebase decouple (#438)

* [Slim] Decouple Api files into separated PHP Classes

This enhancement required for modular testing and code coverage generating.

* [Slim] Define all app routes in SlimRouter PHP Class. Generate new samples

samples/server/petstore/php-slim/lib/Model/StringBooleanMap.php

@@ -0,0 +1,12 @@
+<?php
+/**
+ * StringBooleanMap
+ */
+namespace OpenAPIServer\Model;
+
+/**
+ * StringBooleanMap
+ */
+class StringBooleanMap {
+
+}