This proceeds the work in #2. The review in that PR became focused on "categories" and "redactionPolicies". I'm open this PR here to keep that discussion in tact while moving the review of the basic API design (minus the categories stuff) here.

Highlights

• Adds EventSchema API for validating schemas and events.
• Adds SchemaRegistry to cache validators for all registered schemas. This prevents a memory leak I have been seeing this library.
• Validates all schemas against a meta schema to enforce Jupyter Events schema structure.
• Removed "categories" handling (for now), since this is more of a telemetry use-case.
• Defaults to emitting all events (opposite of Jupyter Telemetry).

Public API changes

• "categories" is no longer used for redacting sensitive information. I
• "allowed_schemas", "allowed_categories", and "allowed_properties", have been removed from the EventLogger.
  • All registered event types will be emitted.
  • The expectation is that consumers will use filters in the logging handlers to listen for specific events.
• EventLogger.record_event has been renamed to EventLogger.emit.