[Merged by Bors] - feat(Algebra/Ring/Ext): prove extensionality lemmas for Ring and similar typeclasses

Mathlib.lean

@@ -454,6 +454,7 @@ import Mathlib.Algebra.Ring.Defs
 import Mathlib.Algebra.Ring.Divisibility.Basic
 import Mathlib.Algebra.Ring.Divisibility.Lemmas
 import Mathlib.Algebra.Ring.Equiv
+import Mathlib.Algebra.Ring.Ext
 import Mathlib.Algebra.Ring.Fin
 import Mathlib.Algebra.Ring.Hom.Basic
 import Mathlib.Algebra.Ring.Hom.Defs