[schema] Application Service Registration meta schema

[V02460]

Sets the meta schema for the Application Service Registration to https://json-schema.org/draft/2020-12/schema. Specifying the meta schema clarifies the semantics of the schema files and allows tools to consume it correctly.

It seems to me that the registration.yaml and namespace_list.yaml are completely disjunct from the other OpenAPI schemas in the data/api/application-service folder. Because the other files follow OpenAPI v3.1.0, I still chose to use JSON Schema version 2020-12, which is the version that OpenAPI v3.1.0 data types and schema objects are based on.

Relevant parts of the OpenAPI v3.1.0 spec that justify the inclusion of the $schema keyword:

[The Schema Object] is a superset of the JSON Schema Specification Draft 2020-12.

Unless stated otherwise, the property definitions follow those of JSON Schema and do not add any additional semantics.

From https://spec.openapis.org/oas/v3.1.0.html#schema-object

Pull Request Checklist

• Pull request includes a changelog file
• Pull request includes a sign off
• Pull request is classified as 'other changes'