Setup GraphSync message types and protobuf encoding

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Sets up a framework for graphsync and implementation of the protobuf encoding Protobuf Graphsync message type #427
  • Status code options built out
  • Request and response types built out, along with all the core logic
  • Conversions between the protobuf and usable types

Using same lib as in beacon for protobuf so no unnecessary deps

Reference issue to close (if applicable)

Closes #427

Other information and links