Add x-typespec-generated to openapi3 emitter

[mikeharder]

Clear and concise description of the problem

The typespec-autorest emitter includes the marker x-typespec-generated in the output:

{
  "swagger": "2.0",
  "info": {
    "title": "(title)",
    "version": "0000-00-00",
    "x-typespec-generated": [
      {
        "emitter": "@azure-tools/typespec-autorest"
      }
    ]
  },

For consistency, I think the openapi3 emitter should also output a marker. Currently, it does not:

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

Checklist

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

[wanlwanl]

Hi @mikeharder, could you share the scenario that uses x-typespec-generated?
@timotheeguerin, what do you think about it?