feat(data/equiv/mul_add_aut): adding conjugation in an additive group (#6774)
feat(data/equiv/mul_add_aut): adding conjugation in an additive group (#6774)

assuming `[add_group G]` this defines `G ->+ additive (add_aut G)`



Co-authored-by: Eric Wieser <da64e90baec5d084ce3d8e05f3b022ae6d118ff5@gmail.com>

leanprover-community/mathlib,leanprover-community/mathlib