Add feature flag to update namespace statistics after snippets update

What does this MR do?

In this MR we introduce the feature flag namespace_snippets_size_stat. When this flag is activated, any update in any snippet repository, will trigger a namespace statistics update (because the snippets_size is part of those stats). This is the same behavior we have for the project repo, wiki pushes, ...

Refs #225615 (closed)

Does this MR meet the acceptance criteria?

Conformity

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