Add `Event.relationsCreated` event to listen for future relations collections
Add `Event.relationsCreated` event to listen for future relations collections

If you ask for relations but none currently exist, we return `null` to avoid the
overhead of many empty relations objects in memory. However, we still want some
way to alert consumers when one _is_ later made, so this event level event
provides that.

Part of https://github.com/vector-im/riot-web/issues/9572
Part of https://github.com/vector-im/riot-web/issues/9485

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