feat(group_placeholder_reassignments): add group placeholder reassignments API

• Add GroupPlaceholderReassignmentsService with DownloadPendingReassignments (GET /groups/:id/placeholder_reassignments, returns the CSV as an io.ReadCloser) and ReassignPlaceholders (POST with a multipart CSV upload, returns the processing message).
• Register the service on Client. The mock, testing.Client and service map are regenerated.
• Tests cover success, path-encoded group IDs, invalid ID types, request option errors and error responses.
• Docs: https://docs.gitlab.com/api/group_placeholder_reassignments/

Closes #2328 (closed)