Add parseable DiscoverySelector representations

[leonard84]

Overview

Before this PR, it was not possible to use the more advanced DiscoverySelectors without writing code. Now, we have a generic way to specify all supported DiscoverySelectors as strings and parse them. This will allow advanced usage scenarios. For example, Gradle can add a Test task parameter to pass a custom selector directly to the platform. This would allow using the IterationSelector when delegating to Gradle in IDEA.

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Definition of Done

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

[leonard84]

@marcphilipp, it looks good, what is missing? Or, are you waiting for me to switch it from draft to rfr?

[marcphilipp]

@leonard84 Yes, it does look good and I will finish it for 5.11 M2.