wireprotov2: send protocol settings frame from client Now that we have client and server reactor support for protocol settings and encoding frames, we can start to send them out over the wire! This commit teaches the client reactor to send out a protocol settings frame when needed. The httpv2 peer has been taught to gather a list of supported content encoders and to advertise them through the client reactor. Because the client is now sending new frame types by default, this constitutes a compatibility break in the framing protocol. The media type version has been bumped accordingly. This will ensure existing clients won't attempt to send the new frames to old servers not supporting this explicit media type. I'm not bothering with the BC annotation because everything wireprotov2 is highly experimental and nobody should be running a server yet. Differential Revision: https://phab.mercurial-scm.org/D4922