Metaschema: support event class and object attribute observables

[rmouritzen-splunk]

Related Issue:

• https://github.com/ocsf/ocsf-schema/issues/960
• https://github.com/ocsf/ocsf-schema/issues/964

Description of changes:

Metaschema changes.

• Add class and object attribute observables.
• Remove hard-coded list of categories from metaschema/categories.schema.json, leaving this to the ocsf-validator. This change makes testing with alternate schemas that may add extra categories easier, as well as making it possible to validate private extensions that contain new categories.
• Detect unexpected top-level properties in object and event class definitions. This was added at this point to detect invalid observable definitions: invalid top-level observable property in event classes, and invalid top-level observables property in objects.