Support anyOf in JSON schema

[MarekSvatos]

OpenAI supports anyOf specification in json schema while using structured outputs.

• https://platform.openai.com/docs/guides/structured-outputs#supported-types
• https://platform.openai.com/docs/guides/structured-outputs#for-anyof-the-nested-schemas-must-each-be-a-valid-json-schema-per-this-subset

JsonAnyOfSchema was added as a new implementation of JsonSchemaElement