Added Customer Deleted Message

We have added a specific Message that is published whenever a Customer is deleted. That means, you can now subscribe to this particular message and do not need to use the generic ChangeSubscription anymore to implement such use cases.

Changes:

• [API] Added CustomerDeleted Message.
• [GraphQL API] Added the following types to the GraphQL schema: CustomerDeleted.

The following changes were introduced in terms of GraphQL SDL:

type CustomerDeleted implements MessagePayload {
  type: String!
}