refactor: use ::class constants

[kenjis]

To fix psalm issues.

See #539

Before:

INFO: PropertyTypeCoercion - src/Config/Auth.php:211:40 - $this->passwordValidators expects 'array<array-key, class-string<CodeIgniter\Shield\Authentication\Passwords\ValidatorInterface>>',  parent type 'array{"CodeIgniter\\Shield\\Authentication\\Passwords\\CompositionValidator", "CodeIgniter\\Shield\\Authentication\\Passwords\\NothingPersonalValidator", "CodeIgniter\\Shield\\Authentication\\Passwords\\DictionaryValidator"}' provided (see https://psalm.dev/198)
    public array $passwordValidators = [
        'CodeIgniter\Shield\Authentication\Passwords\CompositionValidator',
        'CodeIgniter\Shield\Authentication\Passwords\NothingPersonalValidator',
        'CodeIgniter\Shield\Authentication\Passwords\DictionaryValidator',
        // 'CodeIgniter\Shield\Authentication\Passwords\PwnedValidator',
    ];


INFO: PropertyTypeCoercion - src/Config/Auth.php:336:35 - $this->userProvider expects 'class-string<CodeIgniter\Shield\Models\UserModel>',  parent type '"CodeIgniter\\Shield\\Models\\UserModel"' provided (see https://psalm.dev/198)
    public string $userProvider = 'CodeIgniter\Shield\Models\UserModel';


------------------------------
                              
       No errors found!       
                              
------------------------------
2 other issues found.
------------------------------

After:

$ vendor/bin/psalm --show-info=true src/Config/Auth.php 
Target PHP version: 7.4 (inferred from composer.json)
Scanning files...
Analyzing files...

░
------------------------------
                              
       No errors found!       
                              
------------------------------

Checks took 2.78 seconds and used 347.746MB of memory
Psalm was able to infer types for 93.0133% of the codebase

[datamweb]

What about documents?

[kenjis]

Updated the docs.

[datamweb]

Kenjis thanks.
Please fix:
https://github.com/codeigniter4/shield/blob/develop/docs/auth_actions.md#configuring-actions

[kenjis]

Please fix:
https://github.com/codeigniter4/shield/blob/develop/docs/auth_actions.md#configuring-actions

Updated.