httppeer: expose API descriptor on httpv2peer The API descriptor in wireprotov2 is much more expressive than space-delimited tokens and it will be difficult to define methods to query it in all of the ways we'll want to query it. So let's just declare defeat and expose the API descriptor on the peer instance. As part of this, we define a new interface for version 2 peers, fulfilling a TODO in the process. Differential Revision: https://phab.mercurial-scm.org/D4974