Geo primary verification API: Add checksum endpoint to data management API

What does this MR do and why?

This MR adds a new endpoint that recalculate the checksum of the passed model.

References

#537712 (closed)

How to set up and validate locally

1. Enable the feature flag in Rails console Feature.enable(:geo_primary_verification_view);
2. Find a replicable model record and change its checksum value. For example, in the Rails console:

SnippetRepository.find(19).update(verification_checksum: 'abcd')

3. Test the new API endpoint as an admin user with the model name and ID:

# Replace YOUR_ADMIN_TOKEN with an actual admin personal access token
curl -H "Authorization: Bearer YOUR_ADMIN_TOKEN" -X POST \
     "http://localhost:3000/api/v4/admin/data_management/snippet_repository/19/checksum"

4. The object is returned, with its updated checksum 🎉
5. You can also go into the Rails console and check the record. verification_started_at should be close to the timestamp when you made the above request.

MR acceptance checklist

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