wireproto: define content negotiation for HTTPv2 HTTP messages communicate their media types and what media types they can understand via the Content-Type and Accept header, respectively. While I don't want the wire protocol to lean too heavily on HTTP because I'm aiming for the wire protocol to be as transport agnostic as possible, it is nice to play by the spec if possible. This commit defines our media negotiation mechanism for version 2 of the HTTP protocol. Essentially, we mandate the use of a new media type and how clients and servers should react to various headers or lack thereof. The name of the media type is a placeholder. We purposefully don't yet define the format of the new media type because that's a lot of work. I feel pretty strongly that we should use Content-Type. I feel less strongly about Accept. I think it is reasonable for servers to return the media type that was submitted to them. So we may strike this header before the protocol is finished... Differential Revision: https://phab.mercurial-scm.org/D2850