Add support for window requests to engine-gecko

[csadilek]

We now have the API and the implementation for engine-system (added by #1495).

For GeckoView, we're waiting for https://bugzilla.mozilla.org/show_bug.cgi?id=1510314. Once that's resolved, we can send onOpenWindowRequest / onCloseWindowRequest notifications from engine-gecko.

We also need to make a decision which setting to use enable/disable popup blocking (e.g. javascriptCanOpenWindowsAutomatically).

┆Issue is synchronized with this Jira Task

[csadilek]

@vesta0 @bifleming this is marked as "MVP Blocker` but the feature in Fenix isn't: mozilla-mobile/fenix#1486

Should the label be removed (or added in Fenix)?

We're also blocked on GV for this one. So, if this is a blocker we're cutting it pretty close now as this will still require A-C work.

[vesta0]

Thanks for calling that out @csadilek

The reason the Fenix issue wasn't marked as MVP blocker is because we weren't counting on being able to unblock it before MVP which I think you also confirmed. It is still definitely a P1.