Add support for IndexNow

Add support for submitting URLs to IndexNow.

I know there is a feature request for support for Bing, Yandex, and Naver, but each of those may be solved from working inside the individual search consoles similar to how GSC currently works. Supporting IndexNow would require that we add our API key into urlmonitor and would support those search eninges as well as DuckDuckGo from a additional path towards indexing.

https://www.indexnow.org/