Add typing information for parameterized fixtures

What's the problem this feature will solve?

Add typing information for parameterized fixtures to have better support for type checkers.

Describe the solution you'd like

Given a fixture like

@pytest.fixture(params=["--setup-only", "--setup-plan", "--setup-show"])
def mode(request):
    return request.param

what is the type of request? I thought it would be FixtureRequest but this class doesn't have a param object. Thus I suggest to add a generic class ParametrizedFixtureRequest[T] where T is the type of the parameter. Thus in the above example one could annotate request: ParametrizedFixtureRequest[str] and then the type checker would know that request.param is a string.

Alternative Solutions

Additional context

Start by reading the existing FixtureRequest type and the parameterized-fixture example in the issue. Determine how a generic request type could expose request.param to Python type checkers, then verify the proposed behavior with that example and a type checker.