feat(ls): add documentation for OpenAPI 3.1.0 Callback Object

[tim-lai]

Description

Add documentation for OpenApi 3.1 Callback Object

Motivation and Context

Ref: #1989

How Has This Been Tested?

local.

fixture:

openapi: 3.1.0
info:
  title: deref
  version: 1.0.0
paths:
  /a:
    get:
      operationId: aget
      callbacks:
        # the name `onData` is a convenience locator
        onData:
          '{$request.query.queryUrl}':
            post:
              requestBody:
                description: Callback payload
                content:
                  'application/json':
                    schema:
                      $ref: '#/components/schemas/SomePayload'
              responses:
                '200':
                  description: callback successfully processed

Screenshots (if appropriate):

parent callback object:

child pathItem detected (no target needed):

higher level operation.callbacks:

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.