Feature: #107783 - Register Metadata extractors via Interface 

See forge#107783

Description 

Metadata extractors are service classes that are automatically executed whenever an asset / file is added to the FAL storage, or FAL indexing is executed.

Registration of Metadata extractors will happen automatically when the required interface \TYPO3\CMS\Core\Resource\Index\ExtractorInterface is implemented by the class, utilizing autoconfigure tagging by the Symfony Dependence Injection framework.

No further registration is necessary.

Additionally, the class \TYPO3\CMS\Core\Resource\Index\ExtractorRegistry now uses strong type declarations, which should not affect public consumers. The interface remains unchanged in type declarations.

Impact 

Instances of \TYPO3\CMS\Core\Resource\Index\ExtractorInterface will be detected and registered automatically.