Adding the schema for ingest-events so we can perform more validations
against clients (producers + consumers) on this topic.

Specifically, the are 4 known immediate use cases for this:
- Exceptions raised in the ingest consumer can be checked against this schema and DLQed if they do not conform
- Relay CI can check that messages produced on this topic conform to this schema
- Sentry devserver will validate all incoming messages against this schema
- Sentry CI can run these examples against consumer code