Customize the fixture ordering on the same scope level

Could we have one more parameter, e.g. priority (between 0 and 100), for the users the customize the cost of setup/teardown fixture? For example,

@pytest.fixture(scope="session", params=[1, 2, 3], priority=100)
def f1(request):
    pass

@pytest.fixture(scope="session", params=['a', 'b', 'c'], priority=50)
def f2(request):
    pass

def test_f(f1, f2):
    pass

should give

test_order.py::test[1-a] PASSED
test_order.py::test[1-b] PASSED
test_order.py::test[1-c] PASSED
test_order.py::test[2-a] PASSED
test_order.py::test[2-b] PASSED
test_order.py::test[2-c] PASSED
test_order.py::test[3-a] PASSED
test_order.py::test[3-b] PASSED
test_order.py::test[3-c] PASSED

pytest now assumes an equal weighted cost of setup/teardown fixtures. So it reorders the product of fixture combination to optimize the count of setup/teardown time. For the code given before, the current behavior is

pytest_example.py::test_f[1-a] PASSED
pytest_example.py::test_f[1-b] PASSED
pytest_example.py::test_f[2-b] PASSED
pytest_example.py::test_f[2-a] PASSED
pytest_example.py::test_f[2-c] PASSED
pytest_example.py::test_f[1-c] PASSED
pytest_example.py::test_f[3-c] PASSED
pytest_example.py::test_f[3-b] PASSED
pytest_example.py::test_f[3-a] PASSED

The assumption is not pragmatic when

1. The setup/teardown cost of a fixture is much heavier than that of other fixtures
2. The current reordering can have more than 1 solution, while the user cannot determine which ordering will be used (e.g. the cost of ['1-a', '1-b', '2-b', '2-a'] is same as that of ['1-a', '2-a', '2-b', '1-b']).

This enhancement will allow the users to customize the ordering if they want and resolve #2846.

No file, test, or entry point is named in the issue, so begin by locating pytest's fixture parametrization and ordering implementation. Use the proposed priority examples and expected output as the acceptance criteria, and verify that ordering is customizable without breaking fixture setup/teardown optimization.