[FR] Support adding iframes (or other appropriate method) to embed third-party content

**1~3 main use cases of the proposed feature**

As an AppFlowy user who wants to pull in information from elsewhere on the web, I want to be able to embed content using an iframe so I can have it display on a page.

**what types of users can benefit from using your proposed feature**

- Those wanting to embed externally hosted videos (e.g. Loom, YouTube, Vimeo) within a page.
- Those wishing to embed their Google Calendar.
- Those wishing to embed Draw.io or other diagrams.
- Those wishing to display other content stored locally using an iframe or similar

**Additional context**

This would be a good way to address the needs of other feature requests such as:

- [[FR] Integration with Google Calendar #339](https://github.com/AppFlowy-IO/AppFlowy/issues/339)

See video for example using Notion...

https://user-images.githubusercontent.com/83500640/182910639-8da10a71-1aeb-4185-bec8-1abdb4073991.mp4