Allow update of Import::SourceUser records via CSV

What does this MR do and why?

Add CSV export for import source users

This MR adds a CSV import endpoint for the import source user records associated with the provided namespace.

Issue: #455901 (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

How to set up and validate locally

1. Enable the :importer_user_mapping_reassignment_csv feature flag.
2. Enable Direct Transfer and local requests via your Rails console:

ApplicationSetting.first.update(
  allow_local_requests_from_web_hooks_and_services: true,
  bulk_import_enabled: true
)

1. Enable user mapping for Direct Transfer:

Feature.enable(:importer_user_mapping)
Feature.enable(:bulk_import_importer_user_mapping)

1. Import an existing group from your instance into another namespace.
2. Navigate to the placeholders tab of the root namespace of your import destination.
3. Select "Reassign with CSV file".
4. Download the file, and fill in the last two columns with the username or e-mail of the user you're attempting to map to.
5. Confirm that the reassignment is triggered for the appropriate users by refreshing the placeholder list and viewing the placeholder status.