[Specs-cx] for and on behalf of the End User

[nara hideki]

I think the Party element should have "for and on behalf of" information.
Because the current /Contract/Party/Rel has only two type :

 http://openid.net/srv/cx/1.0/#proposer
 http://openid.net/srv/cx/1.0/#acceptor

As =nat talking, if most CX is for contracts between RP and End User,
we should provide the other type or  some relation for proxy signature
for OP Party.

Any ideas.
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