Allow passing params to import API to override project attributes

What does this MR do?

This MR makes it possible to specify project attributes to override when importing a project through the API. Then the values are stored as import_data and overridden at the moment of import.

Why was this MR needed?

This will make it possible for EE customers to override the classification label upon importing a project: https://gitlab.com/gitlab-org/gitlab-ee/issues/4788

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/4788