Allow restricting string field values

[fubuloubu]

Tried to search what the max length is for string fields, couldn't figure that out easily. It seems very useful to limit length to a maximum, to avoid the classic "username is the entire length of the bee movie script" problem, but also seems useful for like a regex match that only certain characters are used.

Might be able to just combine these since regex can also easily specify a max.

[fubuloubu]

I wonder how related this might be when implementing https://github.com/orgs/fief-dev/discussions/384