Generate x-model in OpenAPI output

[lcawl]

🚀 Feature Proposal

Our OpenAPI generator should include support for the x-model extension, which is provided in Bump.sh as a way to indicate objects that should be abbreviated (and typically provide externalDocs links to find the information elsewhere).

Motivation

The Elasticsearch specification currently contains deep details about Query DSL, for example, which we'd rather cover in the Query DSL reference. These deep and recursive objects are currently difficult to display in the API docs. Even when there are enhancements in Bump.sh to better handle this type of depth and recursion, we'll likely still want to point elsewhere for this type of language reference.

Example

#3023 contains overlays that apply the x-model extension.
Applying that extension in the specification rather than in overlays will be less fragile to breaking when the spec changes.