feat: w3-deal protocol

[Gozala]

First pass on the https://github.com/web3-storage/w3filecoin/issues/29 flow.

• Translates structs used by https://github.com/data-preservation-programs/filsigner-relayed/ to IPLD Schema / TS interfaces
• Describes HTTP API flow that could be implemented to allow spade to invoke signing without learning about UCANs
  • We can also route through spade-proxy as an alternative or complimentary approach