As a Watchlist Expiry user, I want watchlist expiry support added to Block API, so that I can watch a user's talk page after blocking in order to monitor for unblock requests.
**Background:** This was split out from T248407. The use-case is you can watch a user's talk page after blocking, say to monitor for unblock requests. The code seems to trace to the `SpecialBlock` class, where it calls `WatchAction::doWatch()` which already accepts an expiry. The parameter should be called `watchlistexpiry` to be consistent with the other APIs (except for the watch API, itself). The parameter type should be "expiry", to be introduced with T248508.
**Acceptance Criteria:**
* Add watchlist expiry support to Block API
*. The parameter should be called `watchlistexpiry` to be consistent with the other APIs (except for the watch API, itself).
* The parameter type should be "expiry", to be introduced with T248508.