Enhance config schema for richer default experiences

Required vs. not required (probably not empty) values

→ 📌 Configuration schema & required values: add test coverage for `nullable: true` validation support Fixed + 📌 [PP-1] Configuration schema & required keys Needs review

Allowed values / enum

Already in core — see this for ckeditor5.plugin.ckeditor5_language for example:

        Choice:
          - un
          - all

And just now: ✨ Add optional validation constraint support to ConfigFormBase Fixed should significantly lower the bar to adopt configuration schema-based validation in admin UIs (i.e. forms) for simple config!