Add media types

[pquentin]

As decided offline:

• Even though this is an endpoint-level property, there's no endpoint-level file, so we use request types and request_media_type/response_media_type keys, as done for URLs
• We use a union of enums and may introduce enums that refer to specific combinations
• We don’t change the metamodel, which stays as string[]
• As a first step, we replicate what is in rest-api-spec. We can fix things later. In particular, for ES|QL and EQL.

This works, as shown by the unchanged schema.json. However, the way I'm getting the request_media_type and response_media_type values is ugly, and I would appreciate a better solution here.

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

No changes detected.

You can validate these APIs yourself by using the make validate target.

[lcawl]

It would be great to ultimately have those text/plain variations appear in the OpenAPI documents too. It's supported per https://swagger.io/docs/specification/v3_0/describing-responses/#response-media-types