feat(data/finsupp/basic): equiv_congr_left (#7755)
As [requested on Zulip](https://leanprover.zulipchat.com/#narrow/stream/116395-maths/topic/Group.20cohomology/near/240737546).
Co-authored-by: Scott Morrison <scott.morrison@gmail.com>
Co-authored-by: Eric Wieser <wieser.eric@gmail.com>
