wireprotov2: advertise set of valid values for requestable fields 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." Differential Revision: https://phab.mercurial-scm.org/D4619