Make default enum values optional

[rohencz-s1]

For some enums we would like to be able to not use default enum values (0 for Unknown and 99 for Other). The way some of the fields are defined now, e.g. activity_id, it is not possible to exclude these default enum values or overwrite the enums definition (which could also be an option). If possible, it should be possible to do the exclusion / overwrite as part of the extension / profile.

Reason why we want this, is to re-use OCSF schema for ingestion, but me more strict in some of the enumerated fields as we know which ones we support and which we consider invalid and thus having them in JSON schema just gives false indication to 3rd party systems (preparing the data for ingestion) that such enum value is supported.

If a separate ticket is required in https://github.com/ocsf/ocsf-server to support this, I can create it.