[Ingest] OpenAPI spec file (#68323)

* All(?) routes
Lots have one or more things wrong but it's a start

* Some Agent-related models & routes.

Plus some examples for request and responses

* Ignore rule requring PascalCase interfaces
It's not an option to change interface name, afaict
lowercase matches the OpenAPI spec/key name

* Remove generated specs

* Fix misplaced schema

Response shape was listed as a request body

Co-authored-by: Elastic Machine <[email protected]>