What is this feature?

Fix the bug creating the stage writer only with the configured branch for export job.

Why do we need this feature?

We uncovered a bug in this other PR #108580. Basically, we always create the writer based on the configured branch and not to write on the target branch that may be passed for the job.

Who is this feature for?

Users of Git Sync / Provisioning.

How

• Add ref option to stage options and use it to stage the writer with the right branch.
• Fix the issue with ref in export worker.
• Add unit tests for export stage options.

Which issue(s) does this PR fix?:

Fixes https://github.com/grafana/git-ui-sync-project/issues/377

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.