[Discuss] OpenAPI spec for registry

From [OpenAPI](https://swagger.io/docs/specification/about/)
> OpenAPI Specification (formerly Swagger Specification) is an API description format for REST APIs. An OpenAPI file allows you to describe your entire API, including:
>
> * Available endpoints (/users) and operations on each endpoint (GET /users, POST /users)
> * Operation parameters Input and output for each operation
> * Authentication methods
> * Contact information, license, terms of use and other information.

If the registry published this, consumers could use the structured format to integrate into scripts and other parts of workflow.

For example, EPM use that file to create the TS types it needs. This would accomplish the goal of https://github.com/elastic/kibana/pull/54955 without requiring the registry to generate them https://github.com/elastic/package-registry/pull/196