code: function SelfSignedJwt(callContext, authority, clientId) {
    this._log = new Logger('SelfSignedJwt', callContext._logContext);
    this._callContext = callContext;
  
    this._authority = authority;
    this._tokenEndpoint = authority.tokenEndpoint;
    this._clientId = clientId;
  }
docs: Constructs a new SelfSignedJwt object.
  @param {object}    callContext    Context specific to this token request.
  @param {Authority} authority      The authority to be used as the JWT audience.
  @param {string}    clientId       The client id of the calling app.