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

Nobody has claimed this yet.

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.

[1]: https://developer.apple.com/documentation/xcode/understanding-the-exception-types-in-a-crash-report#EXCCRASH-SIGKILL
[2]: https://mozilla.github.io/uniffi-rs/udl/callback_interfaces.html

No files or tests are named. Start by reading the Apple background-task and UniFFI callback-interface references, then trace where the Rust SDK commits database writes; done means an implementable callback is invoked before and after store commits, including for iOS consumers over FFI.

Written by the indexing model from the issue text.