Allow specifying a prefix for ConfigExistsConstraint, to enable using it for references to config entities

Problem/Motivation

#3324150: Add validation constraints to config_entity.dependencies added the new ConfigExists constraint. It works well 👍

But it does not support one very common use case: for one config entity to reference another config entity by its ID/name rather than the full config dependency name.

For example:

1. bundle: article in core/profiles/standard/config/install/core.entity_view_display.node.article.default.yml must be prefixed by node.type. for it to be able to use ConfigExistsConstraint
2. format: basic_html in core/profiles/demo_umami/config/install/editor.editor.basic_html.yml must be prefixed by filter.format. for it to be able to use ConfigExistsConstraint
3. field: body in core/profiles/demo_umami/config/install/field.field.node.article.body.yml must be prefixed by field.storage.node for it to be able to use ConfigExistsConstraint
4. et cetera

Hence this blocks:

• #3412361: Mark Editor config schema as fully validatable
• #3421946: [PP-2] Make FilterFormat config entities fully validatable
• #3422872: Add validation constraints to contact.settings
• … and likely several more child issues of #2869792: [meta] Add constraints to all config entity types

Steps to reproduce

N/A

Proposed resolution

1. Update ConfigExistsConstraint to accept an optional prefix
2. Update ConfigExistsConstraintValidator to respect the prefix option.
3. Test coverage, of course

Remaining tasks

User interface changes

None.

API changes

The ConfigExists constraint (added in Drupal 10.1) has a new prefix option which allows it to be used for validating references to a specific config entity. CR: https://www.drupal.org/node/3416240.

Data model changes

None.

Release notes snippet