Add InMemory Execution Engine

[michaelschnyder]

The initial jobbr server should come with an InMemory variant of the execution engine. The Core Engine is already refactored into a separate project in the ForkedExecution Projekt. But there are still a couple of tasks to do

• move core execution to separate gh repo (+ build) and reference it as submodule from the forked execution
• Increase Tests for core component because its now testable on its own. Check if tests are enough
• Create an InProcess Execution Engine in a separate GH / build / NuGet (reference core rt again)
• Improve errro message if no execution engine is installed and point to available execution egines on rtfd.io
• Migrate to .NET 6