[Feature Request] New Custom Field Type Enum

Hey! Thank you very much for docspell, it's an awesome piece of software!

I was thinking that it would be awesome if there was a Custom field Type ` Enumeration`, which acts basically like `Text` but only allows a limited set of values/variants.

One use-case I am thinking about for this is tagging documents as: "relevant for tax ".
That way, I don't have to create a new Tag for each year (`Tax 2019`, `Tax 2020`, `Tax 2021`) which would clutter the Tag collection the long run, but could instead have a `Tax` enum metadata field, with the variants (`2019`, `2020`, `2021`, ...).

Another use-case I could think about is: When storing bank account statements ("Kontoauszug") in docspell. One could add a metadata field "Bank Account", with the account numbers as variants - since each statement relates to one specific bank account.

When changing the set of allowed variants, a validation would have to be run against the custom field's instances, to check that no in-use variant can be removed.