mgr/smb: Update handler to process RGW shares, added validation and error handling

- Add _generate_rgw_share() function for RGW share configuration
- Skip CephFS authorization for RGW shares in cluster assembly
- Configure Samba VFS with RGW-specific parameters:
  - rgw_bucket, rgw_user_id, rgw_access_key, rgw_secret_key
- Validate bucket accessibility using radosgw-admin
- Ensure proper error handling in share creation flow

This enables the handler to generate proper Samba configuration for
RGW-backed shares along with validation and error handling.

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