[web] Platform views support multi-view

[mdebbar]

This is a tracking issue for the work to make platform views support multi-view on the web.

Summary: When creating a platform view, the framework should tell the engine which Flutter View the platform view should be rendered into.

[github-actions]

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.