Ontology: Add custom_properties (e.g. for OWL import)

[jochenchrist]

Summary

Importing an ontology from an external format such as OWL/RDF currently loses any data that the core OSI spec does not model — IRIs, rdfs:label, owl:equivalentClass, annotations, provenance, and so on — because every ontology object is locked down with additionalProperties: false. There is no place to put it, so a round-trip (OWL → OSI → OWL) is lossy.

This PR adds an open custom_properties object to the ontology root, concepts, relationships, and roles. Keys are free-form and values may be any JSON, so importers can preserve foreign data and round-trip it. The rest of each object keeps additionalProperties: false, so custom_properties is the single, explicit escape hatch rather than letting arbitrary keys leak in anywhere.

For semantic-web sources, CURIEs such as owl:equivalentClass or rdfs:label are recommended so the origin of each property stays clear.

ontology:
  - concept:
      name: Person
      type: EntityType
      custom_properties:
        iri: "http://xmlns.com/foaf/0.1/Person"
        rdfs:label: "Person"
        owl:equivalentClass: "https://schema.org/Person"

Design notes

The core spec (logical model) already has a custom_extensions: [{vendor_name, data}] mechanism where data is a stringified JSON blob. I deliberately did not reuse that here:

• OWL/RDF is a standard interchange format, not a single "vendor", so a vendor_name blob is an awkward fit.
• A structured map keeps OWL annotations as real, queryable/validatable JSON instead of an opaque string.

If maintainers would prefer to unify on one extensibility mechanism spec-wide, I'm happy to switch this to the custom_extensions shape instead — flagging the choice explicitly.

Changes

• ontology/ontology.json: add a reusable CustomProperties $def and reference it from the root, Concept, Relationship, and Role.
• ontology/ontology.md: add the field to the four schema tables, a new "Custom properties" section with an OWL example, and a version-history note.
• examples/foaf_owl_import.yaml: a small FOAF/OWL import example exercising custom_properties at the root, concept, relationship, and role levels.

Validation

• ontology/ontology.json is well-formed JSON.
• The new example and the existing examples/flights.yaml both validate against the updated schema (Draft 2020-12).
• Confirmed unknown keys are still rejected everywhere except inside custom_properties.

Language-tagged literals

RDF literals are often language-tagged. The recommended convention is to append the BCP 47 tag to the key after an @ (rdfs:label@en, rdfs:label@fr); @ is not legal in a CURIE local name, so this parses unambiguously. For the cases the flat form cannot express (multiple values in one language, or a non-language datatype such as xsd:date), the docs point at the lossless JSON-LD value-object form. This is convention only; OSI still treats custom_properties as opaque.

[kurtStirewalt]

This is a nice addition!