This commit introduces a new `within_user_updater_transaction` event that's triggered inside the transaction that saves user updates in `UserUpdater`. Plugins can hook into the transaction using the event to include custom changes in the transaction. Callbacks for this event receive 2 arguments:
1. the user being saved
2. the changed attributes that are passed to `UserUpdater`.
There's also new modifier in this commit called `users_controller_update_user_params` to allow plugins to allowlist custom params in the `UsersController` which eventually end up getting passed as attributes to the `UserUpdater` and the new `within_user_updater_transaction` event where they can be used to perform additional updates using the custom params.
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New API is used in https://github.com/discourse/discourse-mailinglist-integration/pull/1.
bb3c05ba0e