Feature: #107871 - Autoconfigure backend avatar providers 

Description 

Backend avatar providers must either use the PHP attribute #[AsAvatarProvider] or be manually tagged in the service container with backend.avatar_provider.

When autoconfiguration is enabled in Services.yaml or Services.php, applying #[AsAvatarProvider] will automatically add the backend.avatar_provider tag; otherwise, the tag needs to be configured manually.

Example:

use TYPO3\CMS\Backend\Attribute\AsAvatarProvider;
use TYPO3\CMS\Backend\Backend\Avatar\AvatarProviderInterface;

#[AsAvatarProvider('my_provider', before: ['provider-1'], after: ['provider-2'])]
final class MyAvatarProvider implements AvatarProviderInterface
{
    // ...
}

Impact 

Backend avatar providers are now automatically registered using the PHP attribute #[AsAvatarProvider]. This improves the developer experience and reduces configuration overhead. The previous registration method via $GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['backend']['avatarProviders'] can no longer be used.

To support multiple TYPO3 Core versions simultaneously, extensions may still implement the legacy array-based registration alongside the new autoconfiguration-based approach.