Update emitter to set extensions at the root of the document

[MaggieKimani1]

Clear and concise description of the problem

This feature request proposes adding extensions defined on a Namespace at the root of an OpenApi document.

Typespec sample:

Playground

@service
@route("/createSong")
@extension("x-custom", "foo")
namespace Suno {

}

Result:

openapi: 3.0.0
info:
  title: (title)
  version: 0.0.0
tags: []
paths: {}
components: {}

Expectation:

openapi: 3.0.0
info:
  title: (title)
  version: 0.0.0
tags: []
paths: {}
components: {}
x-custom: foo

Checklist

• [X] Follow our Code of Conduct
• [X] Read the docs.
• [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[markcowl]

est: 3