Support enum properties in CustomClassDefinition.FromClass

[Metraberryy]

The documentation suggests that CustomClassDefinition.FromClass supports enum properties, but in practice it doesn't; this PR fixes that

[dcronqvist]

Good work on finding this! And even better that you took it upon yourself to create a PR to fix it!!

I agree with your solution, but have made a couple of comments that I'd like for you to look at before I'm happy to merge.

One thing we perhaps need to make sure is also to update the docs to say something about nested enum properties in classes:

They need to be added to the list of available custom types to the loader even if they exist in a class which we are doing FromClass on. So doing a FromEnum and then supplying it to the loader.

[Metraberryy]

I'm unsure of where/what exactly to write in the documentation, but otherwise implemented your suggestions

[dcronqvist]

Nice work! I can create a separate issue for missing documentation on this behaviour and add it myself when I have time. No worries :)