Write spec for RPC Protocol

[yaymukund]

We'd like to expose an RPC protocol that wallets can use to perform actions on behalf of the customer. This will require the customerd rearchitecture.

To close this issue, write out a spec that contains all the required functionality for the RPC requests and responses in a long-term accessible location (maybe the zkchannels-spec repo).

Closed questions (see comments)

1. What kind of RPC? JSON-RPC might be easiest to integrate. Do wallets already use JSON-RPC for other things?
2. Can we reach out to wallet developers directly for feedback?

[yaymukund]

Our requirements are straightforward - we just need to expose the CLI API as RPC commands - and most RPC protocols ought to support it. I think we should go with JSON-RPC because it has libraries [1] in most languages and is used by bitcoind [2] and Zcash [3], so wallet developers should be able to integrate it easily.

The API should mimic the CLI arguments we presently expose.

1: https://github.com/shanejonas/awesome-json-rpc
2: https://en.bitcoin.it/wiki/API_reference_(JSON-RPC)
3: https://zcash.readthedocs.io/en/latest/rtd_pages/zig.html#zcash-integration-guide

[yaymukund]

https://github.com/paritytech/jsonrpc looks like it's actively developed and might be a good fit for Zeekoe's use-case.

Questions:

• Would adding paritytech/jsonrpc increase compile time?
• Would you have to define the interface twice (once for the CLI once for JSON-RPC)? You could use a macro to avoid the duplication, but maybe there's a more elegant way to do it with a trait.

[marsella]

See also: #191 (comment)