Support port mapped iframes in iframe based webviews

[mjbvz]

Problem

Webview port mapping lets extension serve up iframe content from remote resources:

<iframe src="http://localhost:3000"></iframe>

If the extension is running remotely, then port 3000 referes to a port on the remote machine. In this case, we:

1. Intercept the request to http://localhost:3000
2. Open a tunnel to port 3000 on the remote machine
3. Redirect http://localhost:3000 to the tunnel

This is all possible because electron gives us control over web requests inside of <webview> tags. However, as we move to iframe based webviews for the web, we need a new approach

[mjbvz]

My first thought here was to use a service worker to intercept the request for the iframe source. This is how we handle loading of workspace/extension resources inside of iframe based webviews, and I also used a service worker to implement basic port mapping: 29c0595

This however does not work as the iframe is treated more like a unique browser page (so any service worker we register outside the iframe will not see requests for the src or any content inside of the iframe)

[mjbvz]

This is essentially covered by #81131 which allows an extension to resolve the url of any localhost iframes it creates