support plugins as a class

What's the problem this feature will solve?

currently, there is no way to type check your hook functions:

from pytest import Item

def pytest_runtestloop(session: Item): # no type error, it should be Session
    ...

Describe the solution you'd like

expose a base class for plugins to extend, so they can be defined like so:

from typing_extensions import override
from pytest import Plugin

class SomePlugin(Plugin):
    @override
    def pytest_runtestloop(self, session: Item): # type error: signature does not match base method
        ...

Alternative Solutions

Additional context

Start with the pytest_runtestloop hook shown in the issue and review how pytest currently defines and exposes plugin hooks. Determine the scope of a Plugin base class and its hook signatures, then verify that a mismatched session annotation is rejected by a type checker while valid plugin methods remain supported.