Add internal HTTP API for ContentTemplate entity type

Overview

Part of 📌 [PoC] Introduce a `ContentTypeTemplate` config entity Active

The client will need to create, edit, and delete the ContentTemplate entities

The actual $component_tree may still be updated using something like ApiLayoutController but other operations will still need a standard HTTP API

Proposed resolution

Expand ApiConfigControllers and related tests/infrastructure to handle the ContentTemplate entity type

User interface changes

None