Add support for $schema and jsonSchemaDialect

In today's implementation (2.11.0, 3.9.0), the deserialization and serialization of JSON Schemas is effectively hard-coded to 2020-12.

However, starting with OpenAPI 3.1, people can set the $schema property of any given schema to a different dialect (default being documented here.

This entails a couple of things:

1. parsing that keyword first if it exists, or falling back on the document dialect property, or falling back to the default value for the OpenAPI version.
2. if that value is known (like https://spec.openapis.org/oas/3.1/dialect/base using the parsing logic associated with that entry
3. if that value is unknown, we need to load the corresponding schema, and read its schema (recursively) until we find a known value. (note be careful of the security considerations here)

Note: ideally we'd make the OpenAPISchema type generic and allow the caller to provide new registrations of schemas uris with the corresponding parsing logic and data type, but that'd introduce major breaking changes, making the cost prohibitive.

That update will most likely require introducing a JSON Schema service of some kind in charge of mapping the known schemas with their deserialization logic, and having the OpenAPISchema deserializers call into that service.

Beginne damit, die Deserialisierer und Serialisierer von OpenAPISchema nachzuverfolgen und zu identifizieren, wo die Annahme 2020-12 angewendet wird. Definiere den JSON Schema-Dienst rund um die Registrierung von Dialekten, die Auflösung von Fallbacks, die rekursive Behandlung unbekannter Schemas und Sicherheitsbeschränkungen; abgeschlossen ist die Aufgabe, wenn $schema und jsonSchemaDialect den korrekten bekannten oder geladenen Dialekt auswählen, ohne das bestehende Verhalten zu beeinträchtigen.