Group Import API endpoint

What does this MR do?

This MR adds Group Import API endpoint that triggers Group Structure Import process. It's very similar to Project Import API endpoint https://gitlab.com/gitlab-org/gitlab/blob/georgekoltsov/group-import-api/lib/api/project_import.rb#L46-46

Additionally it adds workhorse upload acceleration, that is currently not available in Project Import API. It adds new /authorize endpoint for workhorse to talk to Rails and request file location.

Screenshots

Does this MR meet the acceptance criteria?

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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