Support for UI-less extensions

Our browser would benefit from support of webextensions in WebView2:

• Being able to load our own existing extensions would significantly reduce the amount of effort required to port existing features from our existing extensions to native code.
• In some situations, this will also give us additional capabilities that WebView2 currently does not support (for example modifying the Cookie header of the request).

We realize that some APIs (such as "tabs") don't make sense in the context of WebView2, but since we would control both extension and native code we can make those work (as needed) ourselves. On the WebView2 side of things those could be noops.

Additional notes:

• We envision extension to be shipped with our browser.
• Initial set of extension APIs that's crucial to us:
  • webRequest API and/or declarativeNetRequest API
  • webNavigation
  • tabs
  • storage
  • alarms
• Apple is currently in the process of adding extension support to their WKWebView.
  • Their API is also allowing the host application to communicate such info as tab and window data to the extensions - e.g. _WKWebExtensionTab.h and _WKWebExtensionWindow.h

AB#43658387