Add dependency_proxy_size to GraphQL type

What does this MR do and why?

As part of https://gitlab.com/gitlab-org/gitlab/-/issues/348172 we have added dependency_proxy_size to Namespaces::RootStorageStatistics.

This MR exposes that column via the GraphQL type for the model, so that the frontend can use it to display storage statistics for DependencyProxy to our users.

How to set up and validate locally

1. Ensure you have a Namespaces::RootStorageStatistics record with a dependency_proxy_size value

   rss = Namespace::RootStorageStatistics.find_or_create_by!(namespace_id: root_namespace_id)
   rss.update(dependency_proxy_size: 1234567)

2. Query for that value in GraphQL at http://gdk.host/-/graphql-explorer

   query {
     namespace(fullPath: "path-to-root-namespace-used-above") {
       rootStorageStatistics {
         dependencyProxySize
       }
     }
   }

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.