Fix unresolved OpenAPI references

What does this MR do and why?

Currently, any external references will not resolve correctly because the OpenAPI (Swagger) viewer is rendered in an iFrame, so external requests are blocked due to CORS.

This MR aims to resolve this issue by bundling the spec with its external references into one spec file before passing it to the iFrame.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Create an openapi.yml file containing an external reference:

openapi: 3.0.0

info:
  title: Simple API
  version: 1.0.0

paths:
  /greeting:
    $ref: './greeting.yaml'

2. Create the external reference (e.g. ./greeting.yaml):

get:
  summary: Get a friendly greeting
  responses:
    '200':
      description: A successful response
      content:
        text/plain:
          example: Hello, World!

3. Open the openapi.yml file created in Step 1
4. The contents from ./greeting.yaml should be rendered

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #381047 (closed)