[OpenAPI] Alternative paths/methods and auth info in extensions

[l-trotta]

Fixes #5286

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

No changes detected.

You can validate these APIs yourself by using the make validate target.

[lcawl]

I see content like this in the output for the privileges, which is great IMO:

        "x-req-auth": [
          "Index privileges: `create_index`,`index`,`manage`,`read`,`view_index_metadata`\n",
          "Cluster privileges: `manage_ml`\n"
        ],

For the path variations, I see:

        "x-variations": [
          "<div>\n                      <span class=\"operation-verb get\">GET</span>\n                      <span class=\"operation-path\">/_ml/datafeeds</span>\n                      </div>\n                    ",
          "<div>\n                      <span class=\"operation-verb get\">GET</span>\n                      <span class=\"operation-path\">/_ml/datafeeds/{datafeed_id}</span>\n                      </div>\n                    "
        ],

We can check how this renders, worst case might need to remove the div and span and just use simple backticks here too.