Calculate and Store Storage Sizes of Forks in Root Storage Statistics

What does this MR do and why?

Calculate and store total storage sizes for all forks in the namespace when root storage statistics are recalculated.

This MR populates the public_forks_storage_size, internal_forks_storage_size, and private_forks_storage_size columns on the namespace_root_storage_statistics table whenever we invoke Namespace::RootStorageStatistics#recalculate!.

This is feature flagged off.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/399368

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