Background synchronization / polling notification provider

### Your use case

#### What would you like to do?

Currently, Element X requires the setup of a push notification provider (i.e. UnifiedPush) to get notifications. Without a notification provider set up, no notifications can be received at all. Setup for the push provider adds an extra barrier for users and requires extra setup if the user prefers not to use external services for UnifiedPush. The "old" Element app had a "background synchronization" notification method which effectively polls for new messages given a delay and battery mode parameter. The background synchronization option ensures notifications work out of the box and allows users to keep using polling if preferred for other reasons too. Could "background synchronization" (or a polling notification provider) be added to Element X?

See also https://github.com/element-hq/element-x-android/issues/3031#issuecomment-2620747164.

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

1. Better out of the box experience, easier setup
2. No need to setup a UnifiedPush server or connect to an external service
3. Potential battery savings for people that do not need push notifications

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

I have no specific suggestions except that how background synchronization works in the "old" Element app is great.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

### Are you willing to provide a PR?

No