Jeff has made it clear that this will need to be a plugin (unless it can be done in a theme, which seems likely). History suggests that if you develop the login and hundreds of sites want it, including dozens of enterprise customers, it should become an official plugin in only a few years.
But, if it’s possible to do in a theme, which I think it might be, then arguably discourse already supports the feature, you just need a theme to do it! You can check out Developer’s guide to Discourse Themes or post in #marketplacewith a budget. My guess is that it’s at least $1000 but I could be wrong.