Make ReflectionAttribute generic

[vhenzl]

Resolves phpstan/phpstan#5511.

I used an extension from phpstan-doctrine as an exemple for the implementation of ReflectionClassGetAttributesMethodReturnTypeExtension on but I'm not sure if it's correct as it allows for return types like array<ReflectionAttribute<some random string>>.

[vhenzl]

How do I exclude the code for PHP < 8?

Edit: I figured I have to add a runtime stub for ReflectionAttribute.

[ondrejmirtes]

This is almost perfect, thank you! :)

[vhenzl]

Oh no, I totally missed out getAttributes() on other Reflection classes :(

@ondrejmirtes, since there isn't any common abstraction for reflection classes with getAttributes() and getClass() of the extension interface allows only one class, I should make the existing implementation abstract and then add an extension per each reflection class. Is that correct?

[ondrejmirtes]

@vhenzl It's not. You should make whatever needs to be configurable through the extension constructor, then register it multiple times in .neon.

[vhenzl]

Right, makes sense. Thanks!