Allow iOS consumers to setup for us to call `beginBackgroundTask` and `endBackgroundTask`

[poljar]

On iOS the OS might kill us if we're in the middle of a database write[1] while the OS tries to put the application into the background.

Since all the database handling is happening in the Rust SDK, the client doesn't know when a write might start. If the client is using only the crypto crate, it might guess fairly well when this is going to happen, but otherwise not.

We should provide a trait that people can implement, even over FFI[2], and we'll call the method each time before and after we commit things to the store.

[manuroe]

For reference, we use this interface in the iOS SDK: MXBackgroundModeHandler

The SDK calls one of the callback of this object when it needs to ensure to be able to complete tasks if the app goes in background. Once the task is done , the SDK calls MXBackgroundTask.stop().

The iOS SDK uses it for IO operations: Realm DB, file storage and HTTP requests.