Allow to disable the use of silent auth when using Refresh Tokens

[frederikprijck]

When using our SDK in environments such as electron, where it's common to use the file: protocol, using silent auth (iframes) isn't supported. This is because of the fact that electron will set the window's origin to file://, which is a value not supported by Auth0.

This PR adds a flag, useRefreshTokensFallback, that, when set to false, allows disabling the use of silent auth when using Refresh Tokens. The flag defaults to true.

The implication here is that, as we won't fall back to iframes, the errors regarding refresh tokens (missing_refresh_token or invalid_grant) will be thrown to the end-user, needing them to catch these errors and send the user through an interactive login.

let token: string;
try {
  token = await auth0.getTokenSilently();
} catch (e) {
  if (e.error === 'missing_refresh_token' || e.error === 'invalid_grant') {
    auth0.loginWithRedirect();
  }
}