Add configuration import and export

[inhogog2]

This pull request introduces configuration import and export functionality to the REST API, allowing users to back up and restore all major configuration components (such as load balancers, clusters, endpoints, firewalls, mirrors, and policies) via new API endpoints. It includes the implementation of the handler logic, updates to the OpenAPI specification, and error handling improvements.

Configuration Import/Export API:

• Added new API endpoints /config/export (GET) and /config/import (POST) for exporting and importing configuration as JSON files, including handler implementations in backup.go. Export supports filtering by component, and import supports multipart file upload. [1] [2] [3] [4] [5] [6] [7]
• Added generated operation files for the new endpoints, including parameter binding for filtering export components. [1] [2]

Error Handling:

• Improved error message mapping for configuration import/export, including new messages such as "filename is required", "file is empty", "no configuration file provided", and "invalid json format". [1] [2]