Support for joins across schemas on different hosts

It could be extremely helpful for migrations if Maxscale were capable of joining across schemas on different hosts in a predictable way. This would simplify a lot of migrations and allow us to insert Xpand into environments where it may not originally have been a solution. This also allows us a way to provide this option without serverless analytics for clients not willing to engage that product.

One caveat to consider is that one of the use cases would be to have GIS data in an innodb schema and other data in an Xpand one. So the join would have to be intelligent enough to know which server to pull the functions from.