Geo: Add tests for sync object storage attachments

What does this MR do?

• Move GeoNode#attachments to a better location
• Remove the unused Geo::Fdw::GeoNode#attachments
• Add tests for this scope, especially for object storage settings to prevent e.g. the bug in the issue

Closes #229070 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry => no change in behavior
• [-] 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.