Introduce "Reader" abstraction for project import"

What does this MR do?

Introduce "Reader" abstraction for project import

• a Reader can read JSON files for further processing
• its only implementation would be to read legacy(fat) JSON files

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.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #208870 (closed)