Use fully qualified URIs for default extensions

[benbellick]

Problem

Currently, we're using relative URIs for default extensions. The Substrait tutorial demonstrates using fully qualified/absolute URIs, which appears to be the recommended approach. This makes the appropriate URI to use ambiguous and the current implementation is inconsistent with what the substrait docs suggest.

Solution

Switch to using fully qualified URIs for all default extensions.

Dependencies

This change depends on upstream clarification in substrait-io/substrait#827. Though the linked discussion doesn't technically need to be resolved to address this issue, it provides some good context.