Avoid duplicated code in plugins

[ckunki]

Sonarqube reports duplicated code in plugins csv and holiday-calculator.

Proposal: Create abstract class GenericPlugin<T>.

Open: Where to put class? In api would be fine. logic would require additional dependencies for plugins.

[ckunki]

What do you think, @kaklakariada ?

[kaklakariada]

I also thought about this. The API project is the easiest solution.