Replace opt-in annotation with extension method

[marcphilipp]

Overview

• Replace annotation with boolean-returning method
• Return enum instead of boolean
• Create specialized sub-interface of Extension

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[marcphilipp]

@JojOatXGME I'd appreciate it if you could take a look!

[JojOatXGME]

Looks good. I still kept a few comments to communicate what I was thinking while reading your PR.