[V2] Support grant type other than authorization code (e.g. JWT bearer or client credential)

[stomita]

As the current Connection#authorize() expects the first argument to be a code string returned from authorization server, which cannot be obtained for grant types other than authorization_code. It would be better to support other grant types by accepting the parameter object to pass the server as the first argument instead of code string.

For example, if you are going to do the JWT bearer grant:

const conn = new jsforce.Connection();
const claim = {
  iss: ISSUER,
  aud: AUDIENCE,
  sub: 'user01@mydomain.example.org',
  exp: Math.floor(Date.now() / 1000) + 3 * 60
};
const bearerToken = jwt.sign(claim, cert, { algorithm: 'RS256'});
await conn.authorize({
  grant_type: 'urn:ietf:params:oauth:grant-type:jwt-bearer',
  assertion: bearerToken
});

In client credential grant:

const conn = new jsforce.Connection({
   oauth2: {
      clientId: CLIENT_ID,
      clientSecret: CLIENT_SECRET,
   },
});
await conn.authorize({ grant_type: 'client_credentials' });