[FAU-376] Include slugs in admission requirements data and allow the use of custom admission requirements

[zhyian]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Feature

What is the current behavior? (You can also link to an open issue here)
https://inpsyde.atlassian.net/browse/FAU-376

What is the new behavior (if this is a feature change)?
Slugs are included in admission requirements data, also allowed the use of custom admission requirements(to filter it by slugs)

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
No

Other information:

[amiut]

LGTM, thank you.
In general I'm not sure if it's OK in terms of DDD to include WP-related terminology (slug in this case) in Domain layer, but I have no hard opinion tbh.

[shvlv]

LGTM. As we have link text and link URL, I think slug is pretty okay for our AdmissionRequirement value object.