Request for string codes to be enums

[GinoCanessa]

It would be more ergonomic for string fields values such as these:

public static class AddressTypeCodes {
    public const string POSTAL = "postal";
    public const string PHYSICAL = "physical";
    public const string BOTH = "both";
  }

to be enums

Originally posted by @johnstairs in #2 (comment)

[GinoCanessa]

C# doesn't support string value enums, so doing this requires additional work for the serializer and parser.