Add static method support to ClassDB

* Based on the work done for Variant in the past.
* Added `ClassDB::bind_static_method`
* Cleaned up ClassDB::bind_method to use variadic templates.

This adds support for having static methods in Object derived classes.
Note that this does not make it work yet in GDScript or Mono and, while it works for GDExtension, GodotCPP needs to be updated.

core/object/method_bind.cpp

@@ -83,6 +83,10 @@ void MethodBind::_set_const(bool p_const) {
 	_const = p_const;
 }
 
+void MethodBind::_set_static(bool p_static) {
+	_static = p_static;
+}
+
 void MethodBind::_set_returns(bool p_returns) {
 	_returns = p_returns;
 }