Add empty project migration to Group Migration

What does this MR do?

⚠ behind feature flag bulk_import_projects.

This MR adds empty project migration to GitLab Group Migration tool that ~"group::import" is working on https://docs.gitlab.com/ee/user/group/import/ The idea is to be able to click 1 button to migration any group (with all of it's subgroups and projects) from one GitLab instance to another.

The scope of this MR is to add empty projects, without even a repository, as the plan is to start filling the project up gradually (repo first, then metadata like issues, mrs, updaload, lfs, etc) in future MRs.

Project information is pulled using GraphQL and migrated using Bulk Import's ETL pipeline https://gitlab.com/gitlab-org/gitlab/blob/master/doc/development/bulk_import.md in 3 separate steps:

1. Extract project information from GraphQL
2. Transform / sanitize data in a transformer
3. Load (create the group using Projects::CreateService)

This MR is very similar to what has been done for Group migration https://gitlab.com/gitlab-org/gitlab/blob/f2e1fdbd6ef5e797e96d91f3694a37d34428885e/lib/bulk_imports/groups/pipelines/group_pipeline.rb#L1-L1

Documentation will be updated once feature flag is ready to be turned on.

Screenshots or Screencasts (strongly suggested)

projmig

How to setup and validate locally (strongly suggested)

1. Enable bulk_import feature flag (should be enabled by default) Feature.enable(:bulk_import)
2. Enable bulk_import_projects feature flag Feature.enable(:bulk_import_projects)
3. Create a group that has subgroups and projects in it. Can be done with a rate task on gdk bundle exec rake "gitlab:seed:group_seed[2,root]"
4. Open /groups/new#import-group-pane page and enter your gitlab credentials (url & api scoped token). You do not need 2 gitlab instances to perform this test
5. Select the group you want to import, give it a new name and destination and click Import
6. Wait for it to finish and verify imported result
7. Imported group should have all of the projects present, even if they are empty.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team