Refactor OAuth1 & OAuth2 flows to simplifiy their implementation

[thekid]

The problem with the current implementation is that it only initiates a new authentication flow if there was no previous flow in the session. If we pass ?state=<anything> and a previous authentication flow was present, we would reach the flow error.

$stored= $session->value($this->namespace);
$state= $request->param('state');

if (null === $state || null === $stored) {  // <-- here!
  // Start authentication flow
  // [...]
  return;
}

if ($target= $stored[$state] ?? null) {
  // Fetch access token, redirect to self
  // [...]
  return;
}

// This is reached if $stored= ['previous' => ...]
throw new IllegalStateException('Flow error');

This new implementation starts a new authentication flow in this case.

[thekid]

Released in https://github.com/xp-forge/web-auth/releases/tag/v7.0.0