feat(Algebra/Ring/Ext): prove extensionality lemmas for Ring and simi…

…lar typeclasses (#9511)

Add a file `Algebra/Ring/Ext`, proving `ext` lemmas for all of the ring-like classes (for example, anything from `NonUnitalNonAssocSemiring` to `CommRing`), stating that two instances of such a class are equal if they define the same addition and multiplication operations.
Also prove `ext_iff` variants for each class, and injectivity lemmas for projections from classes to parent classes.

Mathlib.lean

@@ -612,6 +612,7 @@ 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