Refactored and added solutions projections exercises and added new video recording

Refactored and added solutions projections exercises

Refactored projections tests to focus on single-stream projections first, then expand on multi-stream projections. Mixing them in the same exercise brings too many concepts immediately. I decided to refactor the current one to show details and list projections for the single stream and include multi-stream projections as the follow-up exercises. They will come in one of the following pull requests.

See the new version of the exercises:

• projections,
• projections with idempotency handling,
• projections with eventual consistency handling.

Related Pull Requests: #139 , #140

Added link to "CQRS is simpler than you think with C#9 and NET 5!" webinar

Watch it 👇