Geo: Add checksumming endpoint to data_management API

What does this MR do and why?

Add full table checksumming endpoint to the data_management API.

References

Related to #537709 (closed)

How to set up and validate locally

1. Enable the feature flag in Rails console Feature.enable(:geo_primary_verification_view);
2. Re-checksum all records for a given model, for example SnippetRepository:

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

3. In the rails console, check that all records verification_state where updated to 0 (i.e "pending"):

SnippetRepository.all.map(&:verification_state).uniq => [0]

You can do this for any available model, which you can find running Gitlab::Geo::ModelMapper.available_model_names in the Rails console.

MR acceptance checklist

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