Add JWT auth example

[ppcano]

https://stackoverflow.com/questions/46767543/jwt-signing-with-k6/46785899#46785899
https://community.k6.io/t/is-there-a-simple-way-to-sign-http-requests-with-jws/1743/2

https://gist.github.com/robingustafsson/7dd6463d85efdddbb0e4bcd3ecc706e1

From Slack:

Krzysztof Widera: you can check the content https://jwt.io/ or:

function parseJwt (token) {
    var base64Url = token.split('.')[1];
    var base64 = base64Url.replace(/-/g, '+').replace(/_/g, '/');
    var jsonPayload = decodeURIComponent(atob(base64).split('').map(function(c) {
        return '%' + ('00' + c.charCodeAt(0).toString(16)).slice(-2);
    }).join(''));

    return JSON.parse(jsonPayload);
};

@tom-miseur wrote:

atob is technically a web API and therefore not part of the JS ECMAScript2015 / ES6 standard/built-in functions, which is what is at your disposal with k6 JavaScript.
Here's a working example, using our own k6/encoding library to perform the base64-decoding:

import encoding from 'k6/encoding';

export default function (){
  const encodedJwt = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c';

  const result = parseJwt(encodedJwt);
  console.log(JSON.stringify(result));
}

function parseJwt (token) {
  var base64Url = token.split('.')[1];
  var jsonPayload = decodeURIComponent(encoding.b64decode(base64Url, 'rawurl', 's').split('').map(function(c) {
      return '%' + ('00' + c.charCodeAt(0).toString(16)).slice(-2);
  }).join(''));

  return JSON.parse(jsonPayload);
};