Define a new graphsync protocol version (let's use 1.1.0 for now) with new message format

[hannahhoward]

New Message format

syntax = "proto3";

package graphsync.message.pb;

option go_package = ".;graphsync_message_pb";

message Message {

  message Request {
    bytes id = 1;       // unique id set on the requester side
    bytes root = 2;     // a CID for the root node in the query
    bytes selector = 3; // ipld selector to retrieve
    map<string, bytes> extensions = 4;    // aux information. useful for other protocols
    int32 priority = 5;	// the priority (normalized). default to 1
    bool  cancel = 6;   // whether this cancels a request
    bool  update = 7;   // whether this requests resumes a previous request
  }

  message Response {
    bytes id = 1;     // the request id
    int32 status = 2; // a status code.
    map<string, bytes> extensions = 3; // additional data
  }

  message Block {
  	bytes prefix = 1; // CID prefix (cid version, multicodec and multihash prefix (type + length)
  	bytes data = 2;
  }
  
  // the actual data included in this message
  bool completeRequestList    = 1; // This request list includes *all* requests, replacing outstanding requests.
  repeated Request  requests  = 2; // The list of requests.
  repeated Response responses = 3; // The list of responses.
  repeated Block    data      = 4; // Blocks related to the responses

}

Why make a breaking change to protobuf rather than add a new field?

My feeling is it's much easier to do feature detection from the protocol ID than examine both fields in the protobuf and try to figure out which is present.