As a Watchlist Expiry user, I want an expiry type added to ParamValidator, so that we can keep parsing & validation in one place.
Background: See T248407 for context. We now have many APIs that accept an expiry, so we should introduce a new parameter type for this type of value to keep parsing and validation in one place. The watchlist expiry is slightly different than some others in that a blank value means "don't change", not "indefinite". Our refactoring should keep in this mind, as other APIs unrelated to watchlisting (such as the userrights API) treat blank/null expiries differently.
- Add expiry type to ParamValidator