[Gandiva] Add support for functions that can return variable len output

[asfimport]

This is a pre-requisite for ARROW-3459.

Reporter: Pindikura Ravindra / @pravindra
Assignee: Pindikura Ravindra / @pravindra

PRs and other links:

• GitHub Pull Request #2843

_{Note: This issue was originally created as ARROW-3519. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 2843
#2843