pytest_generate_tests based on params and marks from previous parametrize

Issue

In userver, we add indirect params to all tests based on marks using pytest_generate_tests.
(See the full story in #13217.)

The issue arises when we want to apply our implicit param only to some of the test items spawned from an already parametrized test:

@pytest.mark.parametrize(
    "foo, bar",
    [
        ("a", "x"),
        pytest.param("b", "y", marks=pytest.mark.my_mark("hmm")),
        pytest.param("c", "z", marks=pytest.mark.my_mark("what")),
    ])
def my_test(foo, bar, my_fixture, ...):
    # ...


def pytest_parametrize_tests(metafunc):
    # metafunc represents a test function before the split into items.
    # How do we set indirect param "my_fixture" based on marks
    # from one of the cases?

The issue has been previously explored: #4050

Proposed syntax

What I want to discuss here is an implementation of that idea, starting with a possible syntax.

# pytest
@dataclasses.dataclass(frozen=True)
class ProtoItem(pytest.Node):
    # essentially CallSpec2 with a rich API from Node
    params: dict[str, object]
    own_markers: list[Mark]

# usage
def pytest_parametrize_tests(metafunc):
    def param_generator(proto_item: pytest.ProtoItem) -> ParameterSet | Sequence[object] | object:
        if (mark := proto_item.get_closest_marker("my_mark")) is not None:
            return (compute_param_value_from_mark(mark),)
        else:
            return (get_some_fallback_value(),)

    metafunc.parametrize(("my_fixture",), param_generator)

Start with the pytest_generate_tests and metafunc.parametrize entry points described here, then review the related discussions in #13217 and #4050. Compare the proposed ProtoItem and param_generator API with pytest's existing parametrization flow; done means reaching and implementing an agreed design for deriving indirect parameters from per-case marks.