Support lists of strings in for_each_reference

[brikis98]

Description

This is a small work around to allow using lists of strings (rather than lists of interface{}) with for_each_reference. This makes it possible to do transformations on values (e.g., extract keys from a map into a list) and pass the output of the transformation into for_each_reference.

TODOs

Read the Gruntwork contribution guidelines.

• [x] Update the docs.
• [x] Run the relevant tests successfully, including pre-commit checks.
• [x] Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• [x] Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added / Removed / Updated [X].

Migration Guide

[brikis98]

Thanks for the review! Merging now.