Add `system_reboot_required` flag

A single flag saying whether a system reboot is required, only ever true on Home Assistant OS. Supervisor already knows this, so Core surfaces it for the frontend to read. Supervisor does not record which of several causes raised it, so part of this task is deciding whether to pass the reason on as well.

Start by locating how Supervisor exposes the reboot-required state and the Core entry point that surfaces system information to the frontend. Determine the expected flag behavior on Home Assistant OS and whether a cause should also be exposed; done means the frontend can reliably read the agreed state and coverage verifies it.

Written by the indexing model from the issue text.