Add background blur to video calls

### Your use case

#### What would you like to do?
I want background blur (and or custom backgrounds) for video calls, like there are in MS Teams, Webex or Zoom.

#### Why would you like to do it?
Privacy reasons.

#### How would you like to achieve it?
Have a setting like in the before mentioned applications.

### Have you considered any alternatives?

I have considered adding the blur by myself, using third party applications for that. However, they are not that powerful, sometimes hard to set up, not integrated and not always free.

I also considered simply using MS Teams, Webex or Zoom instead, but I'd rather use Element 😄

### Additional context

_No response_

The issue names no files, tests, or entry points. Start by locating Element's existing video-call implementation and media settings, then determine how background blur or custom backgrounds would fit the requested setting. Done means video calls support the requested privacy feature in a documented, usable way.

Written by the indexing model from the issue text.