Support the new `idgen` style in `param_fixture`

[smarie]

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The new **kwargs parametrization style @parametrize(foo=[1, 2]) automatically generates ids foo=1, etc.

This is not yet supported in param_fixture[s], maybe we should add it.

Current workaround is to use @parametrize on an identity @fixture

@fixture
@parametrize(foo=[1,2])
def foo(foo):
    return foo