Add ECS schemas for `kibana.*` and `signal.*` for validation

These schemas are not part of core ECS, but can be used in rules using the alert index. These only exist within Kibana, so we will need to see about building and versioning them from there, similar to how ECS schema is currently built.

Potential sources:
* https://github.com/elastic/kibana/tree/main/x-pack/plugins/security_solution/server/lib/detection_engine/routes/index
* https://github.com/elastic/kibana/blob/main/api_docs/rule_registry.devdocs.json