Provide `INotifier::preloadMany()` to allow implementations to preload and cache data for many notifications at once

[st3iny]

The INotifier::prepare() method is called once for every notification. If an implementation is fetching additional data for each notification this benefits performance. An implementation would then be able to query all data at once instead of doing multiple small queries.

Plan

1. Provide IPreloadableNotifier::preloadMany(INotification[] $notifications, string $language): void (in a new interface).
2. Add the method to the notification manager and only call it when a notifier implements the optional, new interface.
3. Adjust all usages in the notifications app, for example, in EndpointController.