[iOS] Save current exercise progress and restore exercise if need

Support to pause exercise and save its progress so that we can restore it and continue to do exercise in the future

The issue provides no files, tests, or entry points to begin with. First inspect the iOS exercise flow and existing state-management or persistence code, then define how pausing and resuming should work. Done means an exercise can be paused, its current progress is saved, and it is restored for continuation later.

Written by the indexing model from the issue text.