feat: port Topology.Algebra.OpenSubgroup (#2889)

Author: urkud

Mathlib.lean

@@ -1423,6 +1423,7 @@ import Mathlib.Topology.Algebra.InfiniteSum.Ring
 import Mathlib.Topology.Algebra.Localization
 import Mathlib.Topology.Algebra.Monoid
 import Mathlib.Topology.Algebra.MulAction
+import Mathlib.Topology.Algebra.OpenSubgroup
 import Mathlib.Topology.Algebra.Order.Archimedean
 import Mathlib.Topology.Algebra.Order.Compact
 import Mathlib.Topology.Algebra.Order.ExtendFrom