Add snippets_size to RootStorageStatisticsType

What does this MR do?

This MR add the new field snippets_size to the RootStorageStatisticsType GraphQL class.

Refs #224658 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content