wireprotov2: expose rich arguments metadata
Now that we internally store rich metadata about arguments, it makes
sense to make that metadata available to the client. This will allow
clients to validate outgoing command requests before they are sent
over the wire.
Strictly speaking, we should bump the wire protocol version for this
change since it is backwards incompatible. But no client-side code
touches the arguments map and I don't want to incur the work.
Differential Revision: https://phab.mercurial-scm.org/D4618
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