Export rake task

What does this MR do?

This MR adds a new rake task to export project, similarly like we do with import task (!20724 (merged)). This MR is part of our preparation work for introducing ndjson for import/export (#197171 (closed)). This way we can have a tool to easily test and measure export before and after changes.

bundle exec rake "gitlab:import_export:export[root, root, existing_project_for_export, /path/to/file.tar.gz, true]"

Conformity

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

Related to #197171 (closed)

Closes #207847 (closed)