gpart: "gpart --libxo:JP list" duplicates attribute keys

Add leaf-list modifier to attrib as it's possible to have multiple
attributes on a gpart provider.

I purposely made it so that the normal output still says "attrib:" just
so we don't break any scripts people may be using to parse the output,
but the libxo output now says "attribute" just like gpart show will do
once https://reviews.freebsd.org/D53950 is merged.

PR: 291377
MFC after: 1 week
Reviewed by: asomers
Sponsored by: ConnectWise
Differential Revision: https://reviews.freebsd.org/D54080