private/model/api: Generate enum shape functions to retrieve all values

CHANGELOG_PENDING.md

@@ -1,5 +1,7 @@
 ### SDK Features
 
 ### SDK Enhancements
+* `codegen`: Add XXX_Values functions for getting slice of API enums by type.
+  * Fixes [#3441](https://github.com/aws/aws-sdk-go/issues/3441) by adding a new XXX_Values function for each API enum type that returns a slice of enum values, e.g `DomainStatus_Values`.
 
 ### SDK Bugs

private/model/api/shape.go

@@ -862,6 +862,16 @@ const (
 
 	{{ end }}
 )
+
+{{/* Enum iterators use non-idomatic _Values suffix to avoid naming collisions with other generated types, and enum values */}}
+// {{ $.ShapeName }}_Values returns all elements of the {{ $.ShapeName }} enum
+func {{ $.ShapeName }}_Values() []string {
+	return []string{
+		{{ range $index, $elem := $.Enum -}}
+		{{ index $.EnumConsts $index }},
+		{{ end }}
+	}
+}
 `))
 
 // GoCode returns the rendered Go code for the Shape.