Updated Protocol Design w/ CBOR

I've updated the protocol design doc to use CBOR for the request and
response body encoding:

https://github.com/w3c/PFE-analysis/blob/main/design/patch_subset_protocol_v3.md

Also a couple of other unrelated changes:
- Expanded the SparseBitSet description and added a more complex example.
- Added information on sending a GET request for the initial request
(previous version was POST only)