[PP-2] Introduce ConfigEntityForm to standardize use of validation constraints for config entities

Is this superceded by ✨ Add optional validation constraint support to ConfigFormBase Fixed ?

@Dave Reid: no, this is for config entities, that one is for simple config.

Hard-blocked on 📌 Split validation-related methods from FieldableEntityInterface into a separate ValidatableInterface Needs work and soft-blocked on 🌱 [meta] Add constraints to all config entity types Active . At least one of the children of #2869792 needs to be finished before we can do this.

If this also would add #config_target support → , then atypical hybrids (simple config being updated as part of a config entity form) such as #3422872-4: Add validation constraints to contact.settings → would've been able to use #config_target too.