Fix speed issues via better message caching / message auto-downloading

### Your use case

#### What would you like to do?

- Configure how many unread messages are auto downloaded when Element is started
- Configure whether Element auto-downloads messages in rooms that are currently not open/active in the UI
- Should include an option to simply download/cache ALL unread messages

#### Why would you like to do it?

Element can be really really slow, especially if a combination of the below factors is true:
- Encrypted rooms
- Large rooms with many messages
- Slow / high latency Internet connections, for example if Tor is used
- Element hasn't been open in a few days

I've used many messaging apps over the years, most of them behind Tor, and I have not seen any other app that was this slow.

#### How would you like to achieve it?

- Add settings that control whether messages are auto-downloaded before the user actually scrolls to those messages
- Let the user configure if the setting should only apply to the active room, to a specific list of rooms, or to all rooms

### Have you considered any alternatives?

Not sure if there's any alternative, besides using a different client (assuming there is one that does this better).

### Additional context

_No response_