Update graphsync to 2.0 branch - absolute minimum

[hannahhoward]

Goals

Get us to compatibility with the graphsync 2.0 branch with the absolute minimum number of changes. We'd like very much not to stay here -- but this is a starting point. It resolves at minimum the API changes, without addressing more significant issues around message encoding to IPLD

Implementation

• update fake graphsync implementation
• implement changes to encoding message in graphsync extensions with a total hack: encode the message to bytes, then deserialize via IPLD dagcbor + basicnode.Any, and vice versa. It's a mess, but it works.
• this allows us to focus on the more significant changes up through the message types.
  • all of the ToIPLD & Back is hidden inside of message.FromIPLD & Message.ToIPLD
  • presumably by replacing this, we can start the conversion

[codecov-commenter]

Codecov Report

Merging #300 (f75b610) into master (240ff4a) will decrease coverage by 1.51%.
The diff coverage is 61.81%.

❗ Current head f75b610 differs from pull request most recent head 5366ffa. Consider uploading reports for the commit 5366ffa to get more accurate results

@@            Coverage Diff             @@
##           master     #300      +/-   ##
==========================================
- Coverage   67.58%   66.06%   -1.52%     
==========================================
  Files          24       29       +5     
  Lines        3159     3439     +280     
==========================================
+ Hits         2135     2272     +137     
- Misses        678      814     +136     
- Partials      346      353       +7     

Impacted Files	Coverage Δ
message/message1_1/transfer_message.go	11.11% <7.69%> (-13.89%)	⬇️
encoding/encoding.go	41.66% <9.37%> (-21.97%)	⬇️
message/message1_1prime/transfer_message.go	20.00% <20.00%> (ø)
message/message1_1/message.go	55.93% <50.00%> (-3.00%)	⬇️
message/message1_1prime/message.go	53.90% <53.90%> (ø)
message/message1_1/transfer_request.go	59.42% <56.66%> (-10.08%)	⬇️
message/message1_1/transfer_response.go	72.54% <57.69%> (-17.70%)	⬇️
message/message1_1prime/transfer_request.go	68.96% <68.96%> (ø)
message/message1_1prime/schema.go	71.42% <71.42%> (ø)
message/message1_1/transfer_request_cbor_gen.go	28.28% <75.00%> (ø)
... and 6 more