Refactored the CQRS Flow .NET samples to not use MediatR

[oskardudycz]

Refactoring of the CQRS Flow .NET example based on the feedback

• Removed MediatR library not to have a redundant dependency from the external package (no advanced features like pipelines were used)
• Removed marker interface for Commands, Queries and Events to reduce potential ceremony

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This discussion was created from the release Refactored the CQRS Flow .NET samples to not use MediatR.