Remove MediatR package

[boma96]

This PR removes MediatR package and all corresponding references to MediatR in code. This required us to implement mechanism similar to MediatR. I used this opportunity to implement a mechanism that is similar but more closely related to Conductor itself.

List of major changes:

• Implement WorkerInvokerService mechanism which implements basic MediatR-like functionality required for ConductorSharp (this includes Behaviors which were renamed to WorkerMiddleware and Handlers which are referred in code as Workers)
• Replaced IRequest<TOutput> with custom interface (ITaskInput<TOutput>)
• Worker context(workflow id, task id, task name, etc.) is available to middlewares and workers by default.
• Added test for WorkerInvokerService
• Added integration tests which are using TestContainers library to spin up Postgres based Conductor (based on docker-compose-postgres.yaml)

Note that this is breaking change (i.e. will become part of v4, hence v3 branch is created where v3 development will continue).