feat(data/dfinsupp): add `finset_sum_apply` and `coe_finset_sum` (#7499)
feat(data/dfinsupp): add `finset_sum_apply` and `coe_finset_sum` (#7499)

The names of the new`add_monoid_hom`s parallel the names in my recent `quadratic_form` PR, #7417.





leanprover-community/mathlib,leanprover-community/mathlib