Allow access to private files [#3624949]

Problem/Motivation

When storing previews in private://, nobody can access them because they are not managed files (#2346003: Preview image as managed file) so FileDownloadHook never allows access.

Steps to reproduce

Use the pdfpreview formatter on a file field that uses private://. The preview images are then stored in private:// too.

Proposed resolution

Add a hook_file_download that allows access if the user can access the original PDF file.

Remaining tasks

User interface changes

API changes

Data model changes

Issue fork pdfpreview-3624949

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3624949-allow-access-to changes, plain diff MR !21
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