ICRC-30: JSON type syntax for Candid values

[sea-snake]

When communicating Candid data with JSON between web services, there are two options:

• CBOR encode to bytes and send as JSON text value (base64/hex)
• Write Candid data with a JSON syntax

The latter is more accessible to services and dapps outside the IC ecosystem, they don't need to understand and decode CBOR to process incoming data.

A few examples:

• JSON-RPC between dapps and (multi chain) wallets with ICRC-25
• canisters with public HTTP APIs accepting and returning JSON for dapps outside the IC ecosystem that want to query their data.
• Browser messaging and storage of Candid types

This standard standardizes the JSON syntax of Candid type values for above use cases with a strong emphasis on making the data accessible for dapps outside the IC ecosystem without Candid knowledge.

The following technical details differ from other encoding formats of Candid types like CBOR:

• JSON values represent the most straightforward representation of the value.
• JSON values do only partially represent the Candid type, the consuming party is expected to know the Candid types in cases where its needed.
• JSON values for the opt type differ depending on where its used, if possible without losing structural information, null values are left out of the JSON string.
• JSON values are structured to have a minimal footprint and to be straightforward for consuming services outside the IC ecosystem without requiring knowledge of Candid.

The spec can be found at the following gist (until I've found a better place for it): https://gist.github.com/sea-snake/3043a9b83348111326a7699efb97e281