Check and limit snippet repo size

What does this MR do?

Check snippet repository does not exceed the configured snippet size, and blocks push if it does.

We first move EE-specific logic for repository size checking from EE::GitAccess up to GitAccess. Then we call the checking methods check_size_before_push! and check_push_size! in GitAccessSnippet.

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Ref #209867 (closed)