Core: Namespace NetPlay utilities under the NetPlay namespace

Previously there was only one function under the NetPlay namespace,
which is kind of silly considering we have all of these other types
and functions existing outside of the namespace.

This moves the rest of them into the namespace.

This gets some general names, like Player, for example, out of the global namespace.

Source/Core/Core/ConfigLoaders/NetPlayConfigLoader.h

@@ -6,14 +6,18 @@
 
 #include <memory>
 
-struct NetSettings;
-
 namespace Config
 {
 class ConfigLayerLoader;
 }
 
+namespace NetPlay
+{
+struct NetSettings;
+}
+
 namespace ConfigLoaders
 {
-std::unique_ptr<Config::ConfigLayerLoader> GenerateNetPlayConfigLoader(const NetSettings& settings);
+std::unique_ptr<Config::ConfigLayerLoader>
+GenerateNetPlayConfigLoader(const NetPlay::NetSettings& settings);
 }