Add support for services outside of this code base

[02strich]

What changed?

This adds an ExternalService option that allows by other repositories to use the base infrastructure while providing their own service.

Why?

As the code base grows, it becomes useful to enable building system services in seperate repositories (just to manage growth).

The approach still depends on the service type enums listing these external services as option. Such this also adds a definition for the first external service project.

How did you test it?

• built
• run locally and tested manually
• covered by existing tests
• added new unit test(s)
• added new functional test(s)

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