API proposal: AcceptOnlyFromAmong for enum

[KalleOlaviNiemitalo]

 namespace System.CommandLine {
     public partial class ArgumentValidation {
+        public static Argument<TEnum> AcceptOnlyFromAmong<TEnum>(
+            this Argument<TEnum> argument,
+            params TEnum[] values)
+            where TEnum: struct, System.Enum;
     }
 
     public partial class OptionValidation {
+        public static Option<TEnum> AcceptOnlyFromAmong<TEnum>(
+            this Option<TEnum> option,
+            params TEnum[] values)
+            where TEnum: struct, System.Enum;
     }
 }

Same behaviour as in Argument.AcceptOnlyFromAmong(params string[]) and Option.AcceptOnlyFromAmong(params string[]), except:

• These are only applicable for enum types
• The return type is different (but still matches the this parameter)
• The values parameter is TEnum[] rather than string[], and the method calls ToString or similar on each value (not sure what to do about values composed of multiple bit flags)
• Validation is case-insensitive like the Enum.Parse call in ArgumentConverter (see also Extend AcceptOnlyFromAmong with bool ignoreCase = false #2254)

  command-line-api/src/System.CommandLine/Binding/ArgumentConverter.cs

  Line 61 in 209b724

  return Success(argument, Enum.Parse(type, value, true));

Inspired by #1959 (comment)

[KalleOlaviNiemitalo]

Could add the same for Option<TEnum?> as well.