Allow external Hive URIs to use any dataset name or namespace

This is a follow-on to CDK-140. The Hive external dataset URI is "dataset:hive:/*path/:namespace/:dataset", so the namespace and dataset name are always taken from the dataset storage location. This should be something that users can change, for example, "dataset:hive:/.../datasets/ds&namespace=group".

There are two ways to do this:
1. Fix up the existing URIs so that query args can override dataset name and namespace
2. Change the URI structure so that dataset location is an optional argument