[Feature request] Provide option to set default video playback rate

**Is your feature request related to a problem? Please describe.**
For videos, it would be really nice to be able to set a default playback rate.

**Describe the solution you'd like**
An option in “General Settings” to set the default playback rate of videos, if supported.

**Describe alternatives you've considered**
Currently, only way to achieve this on iOS is to open the post, which then shows the video controls where the playback speed can be changed manually.

**Additional context**
It looks like Apple may provide the `defaultPlaybackRate` property to be set at an app level.
[1](https://developer.apple.com/documentation/webkitjs/htmlmediaelement/1631650-defaultplaybackrate)
[2](https://www.macrumors.com/2022/06/09/ios-16-playback-speed-menu/)