Subcribes to a bedrock event using the key of the desired
event. When subscribed, the event handler is both returned,
but also cached internally for unsubscription. This means
the caller of the subscription does not need to worry about
unsubscription since the cache will automatically
unsubscribe handlers on overall teardown.
A cache for bedrock event subscriptions. Stores off a subscription by event key, and upon teardown unregisters all subscriptions.