feat: allow callable for ids on register

[fadrian06]

What kind of change does this PR introduce? (pls check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe below

Description

There was a dead code in Auth::register and Auth::createUserFor because $credentials is not defined, using $userData allows to execute possible callables passed as id.

Does this PR introduce a breaking change? (check one)

• Yes
• No

Related Issue

Before

$auth = new Leaf\Auth;

$auth->register([
  'id' => static fn(): string => uniqid(),
]); // Object of class Closure could not be converted to string

After

$auth = new Leaf\Auth;

$auth->register([
  'id' => static fn(): string => 'abc',
]);

echo $auth->id(); // 'abc';