The function that will be called
whenever the JWT is needed. If the getJwtFn returns the JWT as a
string, it will be converted to Jwt instance automatically.
Returns a Promise resolved with the Jwt instance obtained
by the call to the {@link CallbackJwtProvider.getJwt}. If the
getJwtFn returns the JWT as a string, it is converted to
Jwt instance before returning.
Implementation of IAccessToken that calls the user-provided callback function to get the JWT when requested by the clients.