ext/reflection: Add `getFqn()` method to `ReflectionClass`, `ReflectionConstant`, `ReflectionFunctionAbstract`, and `ReflectionAttribute`

Description

The getName() method on those classes returns a qualified name rather than the fully qualified name.

There are various instances where just a qualified name is not sufficient (e.g. documentation or code gen), and while it is very easy to get the FQN by just prepending \ to the name from getName() this is something that needs to be known to be safe to do.

This maybe also should be added to ReflectionClassConstant as methods would also have this new method automatically due to ReflectionMethod inheriting from ReflectionFunctionAbstract.