Firefox sync doesn't sync AdGuard settings

## The problem

Firefox sync installs AdGuard (= it effectively syncs the "should AdGuard be installed").

But the AdGuard settings on a new Firefox instance are at their defaults.

## Solution: extension settings sync APIs

There supposedly is an [opt-in API that extensions can hook into](https://wiki.mozilla.org/Services/Sync/Addon_Sync#Are_Add-on_Settings_Synchronized.3F), to get their settings automatically synced by the browser:

> For now, if an add-on doesn't preserve settings during Sync, you should contact the add-on's author and request Sync support.

That's why I'm here :)

The link I quoted has broken links, but if I had to guess I guess it boils down to this API: https://developer.chrome.com/docs/extensions/reference/storage/

## Alternative solution

An alternative solution could be to programmatically support importing the settings file (the one you get from "Export settings" button). This could even be just the presence of a file in the addon's install directory? I.e. I could drop this settings JSON file into somewhere?