[change] Added CSV batch download endpoint visibility on Swagger

[stktyagi]

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [x] I have manually tested the changes proposed in this pull request.
• [ ] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [ ] I have updated the documentation.

Reference to Existing Issue

Closes #373.

Description of Changes

Integrated PrivateStorageDetailView within a DRF APIView and applied @extend_schema to enable proper Swagger documentation. This update ensures the CSV download endpoint is both functional and visible in the Swagger UI. Verified the endpoint works correctly and appears as expected in the API docs.

Screenshot

[coveralls]

coverage: 98.635% (-0.05%) from 98.683% when pulling 976f522678b4ff361dfd6575851869cbb1677322 on stktyagi:master into 187e81bf794a2d8d96e344fec5c98823e331c5d2 on openwisp:master.