Step 1 of !28996: Adds models and migrations for Namespaces statistics

What does this MR do?

First part of https://gitlab.com/gitlab-org/gitlab-ce/issues/62214. These new models will be used on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28996.

Models added:

• Namespace::RootStoragestatistics will store the root namespace statistics in an aggregated form.
• Namespace::AggregationSchedule will save a new row when an update to the Namespace::RootStorageStatistics needs to be scheduled.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

#62214 (closed)