File based key management for babyjubjub keys

[jimthematrix]

Similar to Ethereum v3 key files to persist encrypted private keys, save the private key in a JSON file like below:

{
    "crypto" : {
        "cipher" : "aes-128-ctr",
        "cipherparams" : {
            "iv" : "6087dab2f9fdbbfaddc31a909735c1e6"
        },
        "ciphertext" : "5318b4d5bcd28de64ee5559e671353e16f075ecae9f99c7a79a38af5f869aa46",
        "kdf" : "pbkdf2",
        "kdfparams" : {
            "c" : 262144,
            "dklen" : 32,
            "prf" : "hmac-sha256",
            "salt" : "ae3cd4e7013836a3df6bd7241b12db061dbe2c6785853cce422d148a624ce0bd"
        },
        "mac" : "517ead924a9d0dc3124507e3393d175ce3ff7c1e96529c6c555ce9e51205e9b2"
    },
    "id" : "3198bc9c-6672-5ab3-d995-4942343ae5b6",
    "version" : 3
}

[jimthematrix]

fixed by #26

It turns out the keystorev3 format works completely fine with a private key for the Babyjubjub curve. As such we don't need to include the actual key management capability, but instead encourage clients to use an existing solution like that demonstrated in the integration test (/go-sdk/integration-test/e2e_test.go)