Simplify entities registration, validation and parsing in test API

[Artifizer]

Currently, there are two APIs under ./tests/:

POST /entities — accepts plain JSON objects
POST /schema — accepts a JSON object containing a schema plus an additional type_id parameter

With recent changes, a JSON Schema can now be fully identified via its $schema field. As a result, the schema is self-contained, and a separate POST /schema endpoint is no longer necessary.

• Introduce common POST /entities
• validation API must return 'is_schema' and 'is_wildcard' attributes
• parsing API must return 'is_schema' and 'is_wildcard' attributes
• Introduce segment validation (not the whole GTS id, but just segment)