Finally, mouse support is coming to the iPad as an accessibility feature in the new iPadOS, announced today! Our very own Rene Ritchie has confirmed its existence at WWDC after the Keynote was wrapped up.
As of right now, it's unclear exactly what mice are supported, but there have been reports that USB mice and even with Apple's Magic Trackpad. No word on if Bluetooth mice are supported or not.
The developer beta of iPadOS just launched today, and there won't be a public beta until July 2019. The full release fo the software isn't scheduled until Fall 2019.