Add support for JWT Bearer Flow

[AndrewRayCode]

This library does not support the required and best practice authentication mechanism of the "JWT Bearer Flow" https://help.salesforce.com/s/articleView?id=sf.remoteaccess_oauth_jwt_flow.htm&language=en_US&type=5 .

This is the standard way of doing machine to machine OAuth. You have support for setRefreshToken() but this isn't helpful as the standard JWT bearer flow Oauth is what you need to to in order to get a token in the first place. Semi-related, the Ruby restforce gem has similar confusion and missing features around JWT bearer flow.

FWIW here's what it looks like in a Postman pre-request script to get a proper access token with Salesorce, where you build a JWT with claims, sign it with the private key (part of the JWT bearer flow), and gets back an access token that's valid for a few hours:

var navigator = {};
var window = {};

const jwt_header = {
  "alg": "RS256",
  "typ": "JWT"
};
const jwt_claims = {
  "iss": pm.collectionVariables.get("SALESFORCE_CONSUMER_KEY"),
  "sub": pm.collectionVariables.get("SALESFORCE_USERNAME"),
  "aud": "https://test.salesforce.com",
  "exp": Date.now() + 10000
};
const private_key = atob(pm.collectionVariables.get("SALESFORCE_PRIVATE_KEY"));

pm.sendRequest({
  url: 'http://kjur.github.io/jsrsasign/jsrsasign-latest-all-min.js',
  method: 'GET'
}, function (err, res) {
    eval(res.text());

    const signedJwt = KJUR.jws.JWS.sign('RS256', JSON.stringify(jwt_header), JSON.stringify(jwt_claims), private_key);
    // console.log('Assertion to send to Salesforce:', signedJwt);

    pm.sendRequest({
    url: pm.collectionVariables.get("SALESFORCE_TOKEN_URL"),
    method: 'POST',
    header: {
        'Accept': 'application/json',
        'Content-Type': 'application/x-www-form-urlencoded',
    },
    body: {
        mode: 'urlencoded',
            urlencoded: [
            { key: "grant_type", value: "urn:ietf:params:oauth:grant-type:jwt-bearer", disabled: false },
            { key: "assertion", value: signedJwt, disabled: false}
        ]
    }
    }, function (err, res) {
        console.log('Auth err:', err, ' Auth res', res.json());
        pm.environment.set("instance_url", res.json().instance_url);
        pm.environment.set("authorization", res.json().access_token);
    });

});

It looks like this library may be abandonware, are there any forks which add this required auth mechanism?

[jesperfj]

This library is indeed pretty old and is not receiving my attention on a regular basis. But I do check in on it now and then, so it's not complete abandonware :-)

It definitely makes sense to add JWT Bearer Token support. I don't know when Salesforce added support for that. I'll try to get around to it one of these days. Feel free to submit pull requests with test coverage!