Keep an eye on potential omni.ja signing

Over in https://bugzilla.mozilla.org/show_bug.cgi?id=1515712 Mozilla started to implement possible checks for a signed omni.ja. That is currently being a pref corroborator.enabled but we should keep track of this, so we are not surprised once this really hits us.

I think the idea is to sign our omni.ja ourselves once this actually lands.