DRY repository lists for sync candidates.

[madmansn0w]

Should try to facilitate DRY by allowing users to include the use of a yaml processor like yq in the workflow to read from a list of repositories and include them as candidates for file syncing.

I currently do this with a few other workflows and it works well, but I am unable to do this with repo-file-sync-action due to the nature of using .github/sync.yaml as a separate file for configuration. An error is returned in attempting to place yaml handlebars there.

This discussion is (at least somewhat) related to #204.