Don't create one QNetworkAccessManager per forecast query job

Additionally, allow to provide an external QNAM, to reuse one the using application might already have. If we use the internal one, enable HSTS on it as well.

This only covers the forecast jobs, but would also need to be propagated to its subjobs, but the timezone and sunrise ones we can hopefully replace entirely by offline lookup in KI18n and the astronomical calculation code in KHolidays eventually.