Add a plugin hook for handling function/method decorators #9915

Feature

mypy currently has the get_class_decorator_hook plugin hook. I want to propose adding a get_function_decorator_hook and get_method_decorator_hook.

Pitch

Adding these hooks would allow special handling of decorated functions, as would for example be required to solve #9911 using a plugin. mypy currently has special handling for a few decorated such as builtins.property, typing.final and more, but from what I can see it's currently not possible to add additional cases using a plugin.

I have already experimented with this a bit and could probably open a PR in the near future.