mgr/smb: Add CLI command for RGW share creation

- Add 'smb share create rgw' CLI command
- Import rgw module for credential management
- Fetch RGW credentials using radosgw-admin
- Create Share resource with RGWStorage backend
- Handle errors from credential fetching

This enables users to create SMB shares backed by RGW buckets,
similar to 'nfs export create rgw' functionality.

Example usage:
  ceph smb share create rgw mysmb share1 mybucket

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