Add enum() method to Str class

[bertramakers]

Hi,

I propose to add a new enum() method on the Str class. This would make validation of enum properties a lot easier, so we don't need to manually write something like this every time we have an enum property:

Str::make('example')
    ->validate(function (mixed $value, callable $fail) {
        if (!in_array($value, [... allowed values ...])) {
            $fail('must be one of: ' . implode(', ', [... allowed values ...]));
        }
    });

It would also tie in nicely with the enum keyword in JSON schema which is used in OpenAPI specs, so it would also be possible to document the possible enum values in the auto-generated OpenAPI documentation that you are working on in #62

See: https://json-schema.org/draft/2020-12/json-schema-validation.html#section-6.1

(Technically JSON schema allows this attribute on any type, but it seemed most useful to me on string fields for now. It could also be added to other fields later of course.)

[tobyzerner]

Great idea, thanks for the PR! I think I forgot about enums when I was implementing the data type features because they're on a separate page in the Swagger docs 😅

[tobyzerner]

If you have a moment, could you please add this to the docs as well?

[bertramakers]

If you have a moment, could you please add this to the docs as well?

Done in #76 ✅