Add Remote Settings synchronization check from a headless Firefox

[leplatrem]

The browser code to be executed would be something like this:

Services.prefs.setCharPref("services.settings.loglevel", "debug");

const before = Services.prefs.getStringPref("services.settings.last_etag");

const { RemoteSettings } = ChromeUtils.importESModule(
  "resource://services-settings/remote-settings.sys.mjs"
);

await RemoteSettings.pollChanges();
// not thrown!

We could also use await RemoteSettings.inspect(); to look at sync history error etc.

We would have to look at WebDriver / Marionette https://wiki.mozilla.org/WebDriver