default Optional<String> getEnumConstants() {
        return getAsString(HasEnumConstantsUtil.ENUM_CONSTANTS);
    }
Returns a comma separated string of the possible values that this column
may have. If the list of potential values are not constrained, an empty
optional is returned.

@return  list of constant values separated by commas or empty
runtime-parent/runtime-config/src/main/java/com/speedment/runtime/config/trait/HasEnumConstants.java