Support for custom File Reference fields

[nhovratov]

Is your feature request related to a problem? Please describe.
With Content Blocks it is easy to add new Content Types. But for the special table "sys_file_reference", whose type is based on the selected file type, it is not possible in a clean way.

Describe the solution you'd like
It should be possible to a) Override a FileType globally with custom fields / remove fields etc and b) override a FileType locally with overrideChildTca method. How exactly this might look is up to debate.

Describe alternatives you've considered
Alternative right now is to define fields in TCA and reference them in Content Blocks or wrap the File Field around another Collection.

Additional context
The File field is one of the most used types by integrators. Oftentimes the problem arises that they want either to remove some fields like description or link or they want to add custom fields between them.