mgr/smb: Add RGWStorage resource class and update Share to support RGW

mgr/smb: Add RGWStorage resource class and update Share to support RGW

- Add RGWStorage component class for RGW bucket storage backend
- Include fields: bucket, user_id, access_key_id, secret_access_key
- Add password conversion support for credentials
- Update Share class to accept optional rgw field and added convert
  defination
- Update Share validation to support both cephfs and rgw backends
- Ensure exactly one storage backend (cephfs or rgw) is specified

This is the first step in implementing RGW storage support for SMB
shares, similar to the existing NFS RGW export functionality.

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