Merge pull request #33663 from backstage/freben/catalog-model-extensions

Add catalog model layer system with JSON Schema based kind declarations

.changeset/catalog-backend-model-processor.md

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+---
+'@backstage/plugin-catalog-backend': minor
+---
+
+Added `ModelProcessor` that validates catalog entities against the compiled catalog model schemas, and integrated it into the `CatalogBuilder` and `CatalogPlugin`. This processor is only activated if you explicitly add catalog model sources to your backend; there is no functional change for regular catalog usage.

.changeset/catalog-model-layers.md

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+---
+'@backstage/catalog-model': minor
+---
+
+Added a new catalog model layer system that allows plugins to declare and extend catalog entity kinds, annotations, labels, tags, and relations using JSON Schema. The new `createCatalogModelLayer` API provides a builder for composing model definitions, and a `compileCatalogModel` function validates and merges them into a unified model. Built-in entity kinds now include model layer definitions.

.changeset/catalog-node-model-registry.md

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+---
+'@backstage/plugin-catalog-node': minor
+---
+
+Added `provideStaticCatalogModel` that helps provide a static catalog model at startup.

.changeset/scaffolder-common-template-model.md

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+---
+'@backstage/plugin-scaffolder-common': minor
+---
+
+Added `scaffolderCatalogModelLayer`, a Template kind model layer with JSON Schema definition, serving as an example of how plugins can declare their own catalog entity kinds.