AK: Make it possible to not using AK classes into the global namespace

This patch adds the `USING_AK_GLOBALLY` macro which is enabled by
default, but can be overridden by build flags.

This is a step towards integrating Jakt and AK types.

AK/JsonObjectSerializer.h

@@ -262,4 +262,6 @@ ErrorOr<JsonObjectSerializer<Builder>> JsonArraySerializer<Builder>::add_object(
 
 }
 
+#if USING_AK_GLOBALLY
 using AK::JsonObjectSerializer;
+#endif