feat: POD Go library

[rrrliu]

This PR adds a new Go library for the POD specification, and an example HTTP server application. Currently, the following fields are supported, with more support for types incoming.

• String
• Boolean
• Int

POD operations are extremely fast, with signing and verification operations in a 500µs - 2ms range, benchmarked on a 2022 Macbook Pro with an M1 chip.

You can test a deployed version of the HTTP server under gopod.onrender.com. Sample commands for testing.

# Sign a POD
curl -X POST https://gopod.onrender.com/sign \
  -H 'Content-Type: application/json' \
  -d '{
  "entries": {
    "message": {
      "string": "Hello, world!"
    },
    "randomNum": {
      "int": 1231245
    },
    "isValid": {
        "boolean": true
    }
  }
}'

# Verify a POD (you can also try the response from above, also try tampering with the fields)
curl -X POST https://gopod.onrender.com/verify \
  -H 'Content-Type: application/json' \
  -d '{
    "entries": {
      "isValid": {"boolean": true},
      "message": {"string": "Greetings from Go"},
      "randomNum": {"int": 1231245}
    },
    "signature": "a465986417d2cdc0138123914ddcaf2c00dbd8623498e76515b610d434f256221dc11470c0a195fdbc1f4b9d2ab4144c4c972cd272e08bacb40bb5c0e8076d04",
    "signerPublicKey": "c433f7a696b7aa3a5224efb3993baf0ccd9e92eecee0c29a3f6c8208a9e81d9e"
}'

# Sign a POD, and get its Zupass URL
curl -X POST https://gopod.onrender.com/zupass \
  -H 'Content-Type: application/json' \
  -d '{
  "entries": {
    "message": {
      "string": "Hello, world!"
    },
    "randomNum": {
      "int": 1231245
    },
    "isValid": {
        "boolean": true
    }
  }
}'

[robknight]

I am fairly sure that this will not work consistently with PODs serialized using our JavaScript library.

The JavaScript POD library can emit multiple representations for each POD value type, and so a compliant deserialization library must, in theory, support all of them. In the case of int values, this is not just a theoretical concern, as the default behaviour of the JavaScript POD library is to change the representation depending on the size and sign of the integer.

• If the integer is below 56 bits in size, then the JSON number type is used
• If the integer is above 56 bits in size, and positive, then a hexadecimal string is used
• If the integer is above 56 bits in size, and negative, then a decimal string is used

The difficulty of implementing compliant deserialization libraries was one of the points of discussion when this was originally implemented. I suspect that our Rust implementation is also not compatible with the JavaScript one.