Sign API endpoint

[cbermudez97]

Original Issue: fibercrypto/skyxcommons#5

Implement [POST] api/sign

[POST] /api/sign
Should sign given transaction with the given private key Body:
{
 // Private keys, which were returned by the
 //  [POST] /api/wallets.  Multiple keys can be used
 // for transactions with multiple inputs
"privateKeys": [
"string" ],
 // The transaction context in the blockchain
 // specific format
 //  [POST] /api/transactions or  [PUT] /api/transactions
"transactionContext": "string"
}

Response:

{
 // Signed transaction, which will be used to broadcast
 // the transaction
 // [PUT] /api/transactions/broadcast
 “signedTransaction”: “string”
}

Python implementation

@api.route('/sign', methods=['POST'])
def post_sign():
    """
    Sign transacction with private key
    """
    if not request.json:
        return make_response(jsonify(build_error('Invalid Input Format', error_codes.badFormat)), 400)

    if "privateKeys" not in request.json:
        return make_response(jsonify(build_error('Invalid Input Parameters', error_codes.missingParameter)), 400)

    if "transactionContext" not in request.json:
        return make_response(jsonify(build_error('Invalid Input Parameters', error_codes.missingParameter)), 400)

    private_keys = request.json['privateKeys']
    signedHashHex = request.json['transactionContext']
    for secKey in private_keys:
        signedHashHex = sign_hash(signedHashHex, secKey)
    return signedHashHex