Add CSV format to group dependency export

What does this MR do and why?

Update the segmented dependency export to handle multiple export formats. This is done by moving the logic for writing and combining parts from the SegmentedExportService to the exporter for the given format.

This MR also includes API and frontend changes to request a group dependency export as CSV.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Export dependency list in CSV format (#435843 - closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Screen_Recording_2025-03-03_at_12.32.41_PM

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. gdk restart rails-background-jobs to ensure sidekiq is reloaded
2. Navigate to a group which has dependencies
3. Click Export -> Export as CSV
4. Go to <gdk_url>/rails/letter_opener/
5. Download the export using the emailed link