[meta] Config validation for a more reliable Drupal + reliable Recipes from the start

Problem/Motivation

1. Numerous bugs are caused by code expecting config to contain certain key-value pairs and expect those values to be a certain shape, but they aren't
2. 👆 This often leads to broken update paths!
3. Very often working on core is slowed down by invalid configuration in an existing test or new tests.
4. Sites using configuration management often have to deal with hard-to-understand config diffs, and worse, when using modules like https://www.drupal.org/project/config_split, chances are that the resulting config is invalid.
5. Recipes will install modules/themes, apply other recipes, but will primarily be installing and updating configuration. Combining many recipes == combining lots of config. This results in a Cambrian explosion of config, and will make it very probable that a combination of recipes results in a broken ("misconfigured") site. UNLESS we make config validatable, and validate config after applying a recipe: #3405328: [meta] Make recipes safer to use in the real world by supporting config validation and rolling back a broken recipe
6. We'd like to be able to enable new use cases for Drupal, such as decoupled admin UIs, but that requires the ability to safely modify configuration via APIs, and hence needs thoroughly validatable config. That would unblock #2300677: JSON:API POST/PATCH support for fully validatable config entities.

All of these can be solved by getting Drupal core to 100% validatability! And after core, contrib.

Steps to reproduce

See above.

Proposed resolution

1. Add missing infrastructure.
2. Add validation constraints to all config.

Remaining tasks

🙏 Use the tags Configuration schema, validation on all issues.

Core-wide patterns

1. ❌ 100% of simple config should become validatable: #2952037: [meta] Add constraints to all simple configuration
2. ❌ 100% of config entity types should become validatable: #2869792: [meta] Add constraints to all config entity types
3. ❌ 100% of simple config with admin UIs/forms in core should support validation constraints: #3384790: Update all remaining ConfigFormBase subclasses in Drupal core to use #config_target
4. ❌ Make Drupal updates more reliable: #3415061: Make update paths for config more reliable: check validation constraints in update path tests

Missing infrastructure

• ❌ Make PrimitiveTypeConstraintValidator and ConfigSchemaChecker consistent with each other: #3421993: PrimitiveTypeConstraintValidator vs ConfigSchemaChecker: test coverage to establish the status quo
• ❌ It is today impossible to detect whether config has changed, because it doesn't know which order changes are meaningful vs meaningless. Missing infrastructure: #3230826: Expose API to sort arbitrary config arrays: TypedConfigManager::getCanonicalRepresentation() should override its parent
• ❌ Stop importing configuration known to be invalid: #2414951: Validate configuration schema before importing configuration
• ❌ #3427106: Config validation: config entities should get the same validation errors when validated as plain config vs ConfigEntityAdapter
• ❌ Do for config entity types what #config_target did for simple config forms: #3231342: [PP-2] Introduce ConfigEntityForm to standardize use of validation constraints for config entities
• ✅ Require langcode: … for translatable config only: #3427564: Require `langcode: …` only for simple config that contains translatable values
• ✅ Increase visibility of this unofficial initiative to all core contributors to rally the entire community and prevent reductions in coverage: #3422641: Add a "Validatable config" tests job to GitLab CI to help core evolve towards 100% validatability
• ✅ Explicitly declaring which config schema types are fully validatable using a no-op FullyValidatable constraint: #3364109: Configuration schema & required values: add test coverage for `nullable: true` validation support
• ✅ Explicitly optional values: #3364109: Configuration schema & required values: add test coverage for `nullable: true` validation support
• ✅ Explicitly optional keys: #3364108: Configuration schema & required keys
• ✅ Introduce #config_target for ConfigFormBase: #3382510: Introduce a new #config_target Form API property to make it super simple to use validation constraints on simple config forms, and adopt it in several core config forms
• ✅ Introduce ConfigEntityValidationTestBase to gradually be confident about the validation of each config entity type: #3324150: Add validation constraints to config_entity.dependencies
• ✅ … but prevent contrib/custom module disruption: #3402168: Follow-up for #3361534: Config validation errors can still occur for contrib modules, disrupting contrib
• ✅ Validate config during core tests: #3361534: KernelTestBase::$strictConfigSchema = TRUE and BrowserTestBase::$strictConfigSchema = TRUE do not actually strictly validate

User interface changes

TBD

API changes

TBD

Data model changes

TBD

Release notes snippet

All config in Drupal core is now fully validatable.