Expose container_registry_size via GraphQL

What does this MR do and why?

So that ProjectStatistics#container_registry_size can be used in the Usage Quotas page, we expose the attribute via GraphQL

Screenshots or screen recordings

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How to set up and validate locally

1. Visit the GraphiQL explorer at http://gdk.test:3000/-/graphql-explorer
2. Execute the query:

   query {
     project(fullPath:"path/to/your/project") {
       statistics {
         containerRegistrySize
       }
     }
   }

MR acceptance checklist

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