feat: add only option for authentication routes configuration

[dimtrovich]

Description
This pull request adds the ability to specify only the authentication routes to include via a new only option in the routes() method.
In fact, if we only want the login, it would be easier to use ['only' => ['login']] than ['except' => ['register', 'logout', 'magic-link']]

Changes in Auth.php

• New only option: Allows specifying a list of routes to include (opposite of except)
• Mutual validation: Added a check to prevent the simultaneous use of only and except

Benefits of this change

• Improved flexibility - Complements the except option by offering a reverse approach (whitelist vs. blacklist)
• Intuitive API - Allows for a more concise configuration when few routes are needed

Usage examples

// Include only the login and logout routes
service('auth')->routes($routes, ['only' => ['login', 'logout']]);

// Exclude magic-link routes (opposite behavior)
service('auth')->routes($routes, ['except' => ['magic-link']]);

// Error - these two options

**Checklist:**
- [ ] Securely signed commits
- [ ] Component(s) with PHPDoc blocks, only if necessary or adds value
- [ ] Unit testing, with >80% coverage
- [ ] User guide updated
- [ ] Conforms to style guide

[michalsn]

This looks useful, thanks.