feat(*): generalize to and add distrib_smul instances (#16132)
This PR builds on #16123 by adding `distrib_smul` (and `smul_zero_class`) instances for:
* `finsupp`
* `add_monoid_algebra`
* `polynomial`
* submodule quotients
* scalar multiplication by `ℚ`
It also generalizes some results by weakening `distrib_mul_action` to `distrib_smul`. The choice of instances and generalizations is based on which are necessary to solve instance diamonds in `splitting_field`, so I probably missed a lot of additional changes. Since I'm not so interested in generalizing everything at the moment, I'd rather leave missing instances to follow-up PRs.
Co-authored-by: Eric Rodriguez <ericrboidi@gmail.com>
