changesetdata, manifestdata, and filedata all allow the caller to
specify what data fields to request.
Data fields are extensible and may evolve over time. In order to
prevent clients from making requests for fields that are not
available, the client needs to know what fields are available.
This commit teaches the server to declare a set of "valid values"
for wire protocol command arguments. That set of values is exposed
in the command's capabilities descriptor. The changesetdata,
manifestdata, and filedata commands all declare their set of
available "fields."