Enable user to not include projects when migrating group - backend

Problem

When user migrates a group with GL Migration, on gitlab.com they cannot choose not to migrate project relations.

Proposed solution

Add a possibility for user to choose in UI when they migrate group(s) to disable migrating any projects.

User can choose for each group separately not to migrate its projects.

User can select few/all groups and choose not to migrate projects of this groups with one global toggle, see designs.

The default is migrating group(s) with projects.

User should be able to not include projects if they want to also via API.

Technical details

1. Add a new column to the bulk_import_entities table to store the selected option
2. Update BulkImports::Groups::Stage to skip ProjectEntitiesPipeline if the user chooses not to migrate projects
3. Update API::BulkImports to accept the new option.
4. Update Import::BulkImportsController to accept the new option
5. Update BulkImports::CreateService to accept the new option
6. Update the API to expose the selected option API::Entities::BulkImports::Entity
7. Update BulkImports::Groups::Pipelines::SubgroupEntitiesPipeline to propagate the option to subgroups

See also #383332 (comment 1208815057) for enabling frontend .

[ ] Update documentation https://docs.gitlab.com/ee/user/group/import/