CDK-469: Use schema URIs in create command

This updates the create command to pass Schema URIs to DatasetDescriptor, which will set them using the URI option for Hive tables and avoid [CDK-469](https://issues.cloudera.org/browse/CDK-469) for schemas longer than 4k.
This isn't a standard scheme, but it is consistent with what `DatasetDescriptor` does. In fact, this allows us to pass the URI directly to `DatasetDescriptor`, which was part of the problem this is fixing.

I also think that using files from the classpath should be explicit. When we get a location that isn't on the file system, we don't really know whether the user mis-typed a path (e.g., `./` instead of `../`), was in the wrong directory, or actually intended to resolve the file from the classpath. It's easy to leak avsc files into runtime jars, which means it would be an easy mistake to use the wrong schema. The reverse could also be true, where the user as a test update in the current dir and actually intends to use the resource version. The "resource" prefix avoids these and I think is more in line with the principle of least surprise.