Create endpoints for Group import UI

What does this MR do?

We want to have a user interface that will allow the user to import a group from a gitlab export. This is intended to match the functionality that we have in the API and the frontend will be the same as the project import that we currently have.

Addresses the backend component of: #211807 (closed)

1. Creates two new endpoints on the GitlabGroupController to be used by the frontend in this MR: !29271 (merged). These two endpoints match the interface for the API.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation ([if required](https://docs.gitlab.com/ee/development/documentation/workflow.html#when-documentation-is-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

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