Fixed metaschema validation

This PR fixes the logic for metaschema validation, so that it properly uses the Reader's definition of the loaded JSON (via contents) rather than loading it directly from a file. This allows it to be tested properly with an in-memory version.

Fixes #15

Note

Because the concrete Reader walks schema definitions, the data loaded is different than purely validating the metaschemas against their JSON definitions. This has pros and cons:

On the one hand it means this change will appropriately catch a few scenarios that were previously uncaught.
On the other hand the error messages might be hard to understand.

For example, this now catches:

TESTING: JSON files match their metaschema definitions
   FATAL: File at events/network/file_activity.json does not pass metaschema validation. Error: 'optional' is not one of ['recommended', 'required'] at JSON path: '$.attributes.connection_info.requirement'

Which is correct, we recently disallowed primary attributes to be optional, and the server believes that to be the case too:

But at face value, if I were new to OCSF and saw that error, I might be confused, because here's how connection_info in events/network/file_activity.json is defined:

    "connection_info": {
      "requirement": "optional"
    },

It's not obvious that the attribute is primary because the base network event defines:

    "connection_info": {
      "group": "primary",
      "requirement": "recommended"
    },

ocsf / ocsf-validator

Fixed metaschema validation #13

Note