feat: add onBeforeMount callback to createIframeUi options

[bohuslavsemenov]

Overview

This PR introduces an onBeforeMount callback to the createIframeUi function. The main motivation is that the allow attribute must be set before the iframe is injected into the DOM, otherwise browser will not include permission policies specified in it.

Manual Testing

1. Go to the wxt-demo project.
2. Add the following code to entrypoints/iframe-src/index.html

navigator.mediaDevices.getUserMedia({
  audio: true,
  video: true,
});

3. Add the onMount callback to set allow attribute to iframe.content.ts:

onMount: (wrapper, iframe) => {
  iframe.setAttribute('allow', 'camera;microphone');
}

4. Go to google.com and try to refresh the page. You will get inconsistent permission prompts (Actually, they will work only when iframe is hot reloaded or on "reload iframe" click in browser context menu).
5. Change onMount to onBeforeMount and it will prompt you every time you refresh the page.

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