mgr/smb: Add test coverage for RGW share functionality

Add test cases for RGW-backed SMB shares

Key changes:
- RGWStorage creation, validation, and serialization
- Edge cases: None values, mutual exclusion

Signed-off-by: Shweta Sodani <ssodani@redhat.com>
Signed-off-by: Avan Thakkar <athakkar@redhat.com>