postgres automatic table list partitioning
I haven't heard of any plans to add such a feature.

You can easily create a BEFORE INSERT trigger that creates partitions on demand, but of course that trigger has to run for each row inserted, which means a certain performance impact.

Using a default partition like you suggest is a solution with better performance, but it requires later explicitly moving the data to a different partition, which can be painful.

If you expect new countries to pop up all the time, maybe the trigger is the way to go. If it happens once in a blue moon, I might prefer the second way.