[C++] Add fallback for shorthand Substrait URIs without scheme

[asfimport]

At the moment there is no agreed URI base for the standard Substrait functions. The Substrait consumer is currently expecting https://github.com/substrait-io/substrait/blob/main/extensions/FILENAME.YAML but Isthmus is currently generating /FILENAME.YAML.

Although this is a short-term problem I could imagine it popping up again from time to time when users are developing their own functions. So, as a workaround, I suggest that any URI equal to the empty string or "/" should be treated as a "shortcut" and we fall back to name-only matching.

Reporter: Weston Pace / @westonpace
Assignee: Weston Pace / @westonpace

PRs and other links:

• GitHub Pull Request #14143

_{Note: This issue was originally created as ARROW-17736. Please see the migration documentation for further details.}

[asfimport]

Weston Pace / @westonpace:
Issue resolved by pull request 14143
#14143