Add "platform extension" concept

[rmouritzen-splunk]

The extensions defined in the core schema (in the https://github.com/ocsf/ocsf-schema repo) are special-cases and should be presented uniquely. This can be entirely done in the OCSF Server, which is why this an issue in this repo. No changes are needed in the ocsf/ocsf-schema repo.

Any extension defined in the core schema (in the ocsf/ocsf-schema repo) will be a "platform extension".

Solution TBD. The presentation and naming of these "platform extensions" is not offered here yet.

cc: @pagbabian-splunk

[pagbabian-splunk]

I favored this approach initially but it was quicker to use the existing extension mechanism at the time.
In addition to having a separate section on the left column of the browser, the metaschema location may need to be updated: today the platform extensions are in the extensions subfolder under schema. If the server isn't started up with the extensions folder explicitly included, the platform extensions won't show up. That is desired for external extensions but for core platform extensions, it may be better to locate them distinctly in a well known location.