[OpenAPI] Add securitySchemes

[lcawl]

🚀 Feature Proposal

The OpenAPI documents are missing securitySchemes and security declarations.

They could perhaps be added similarly to the info in https://github.com/elastic/elasticsearch-specification/blob/main/compiler/src/steps/add-info.ts

NOTE: I believe that you could have the same list of securitySchemes for both documents, but the security list for the serverless document would not include basicAuth.

Motivation

Relates to elastic/docs-content#192

Example

In the Kibana openAPI document, it looks like this:

security:
  - apiKeyAuth: []
  - basicAuth: []

...

  securitySchemes:
    apiKeyAuth:
      description: |
        These APIs use key-based authentication. You must create an API key and use the encoded value in the request header. For example: `Authorization: ApiKey base64AccessApiKey`
      in: header
      name: Authorization
      type: apiKey
    basicAuth:
      scheme: basic
      type: http

[lcawl]

As a work-around I've created #3198