Include snippets size in project statistics

What does this MR do?

This MR adds the necessary logic to calculate the total snippets size of project's snippets and store the value in the ProjectStatistics value.

Refs #223712 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content