Added solution for projection exercise with eventual consistency

workshops/introduction_to_event_sourcing/src/solved/10_projections_single_stream_eventual_consistency/README.md

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+# Exercise 09 - Projections Idempotency
+
+With the [Database](./tools/database.ts) interface representing the sample database, implement the following projections:
+
+1. Detailed view of the shopping cart:
+   - total amount of products in the basket,
+   - total number of products
+   - list of products (e.g. if someone added the same product twice, then we should have one element with the sum).
+2. View with short information about pending shopping carts. It's intended to be used as list view for administration:
+   - total amount of products in the basket,
+   - total number of products
+   - confirmed and canceled shopping carts should not be visible.
+
+Add event handlers registrations in [projections.exercise.test.ts](./projections.exercise.test.ts) calling [eventStore.subscribe](./tools/eventStore.ts) method.
+
+If needed expand existing classes definition.
+
+Track and implement proper eventual consistency handling in projection event handlers. Change documents definitions, projection logic or database if needed.
+
+Read more about projections in my article:
+
+- [Guide to Projections and Read Models in Event-Driven Architecture](https://event-driven.io/en/projections_and_read_models_in_event_driven_architecture/?utm_source=event_sourcing_nodejs&utm_campaign=workshop)
+- [A simple trick for idempotency handling in the Elastic Search read model](https://event-driven.io/en/simple_trick_for_idempotency_handling_in_elastic_search_readm_model/?utm_source=event_sourcing_nodejs&utm_campaign=workshop)
+- [Dealing with Eventual Consistency and Idempotency in MongoDB projections](https://event-driven.io/en/dealing_with_eventual_consistency_and_idempotency_in_mongodb_projections//?utm_source=event_sourcing_nodejs&utm_campaign=workshop)