Add endpoints for the group export UI

What does this MR do?

Addresses the backend requirements for #211805 (closed)

1. Creates two new endpoints on the GroupsController to be used by the frontend in this MR: !28573 (merged). This matches the existing interface for the API.

• export to start the export of a group
• download_export to download an already existing export file

2. Update the API endpoint to delete existing exports when creating new ones (this is how this works for project export currently)

Note that sending an "export complete" email to the user will be tackled in this issue: #211806 (closed)

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