Add periodic CBF sync in background process

Most blocks will not be relevant to the user, but all compact block filters need to be checked. To ensure the bandwidth requirement is low when the user opens the app, we will probably want to check these in a background task via [`BGProcessingTask`](https://developer.apple.com/documentation/backgroundtasks/bgprocessingtask) at good times (i.e. phone is plugged into the charger and on WiFi). If this is done around once per week, the task should have plenty of time to run before getting killed by the OS. Some prerequisites before we'd be able to do this:
- Access to the public descriptors when the phone is locked, so we may create a `Wallet`.
- #351 to help record if the background task is actually running on live devices.

Demonstrating this should be helpful for developers looking to integrate compact block filters, and is just about as "cutting edge" as we can get with the current capabilities of iOS