Add subscriptions

[aimed]

Implementing subscriptions is a necessary step to properly implement CQRS.

If we adhere to all CQRS best practices, executing a mutation (command) should not return a value (see e.g. https://en.wikipedia.org/wiki/Command%E2%80%93query_separation). Instead the view (query) should be updated after receiving an event.

The flow we try to achieve is something like:
mutation -> create event -> update stores -> inform subscribers