Allow bulk print to separate files

Problem/Motivation

When printing entities in bulk as PDF, I would prefer to have them as separate files instead of all grouped in a single file.
I've gotten most of the way there, but I'm struggling to serve the zip file as a StreamedResponse instead of storing it on the file system.
I am also getting an error about Deprecated function: ZipArchive::open(): Using empty file as ZipArchive is deprecated in Drupal\Core\Archiver\Zip->__construct(), which I can't figure out how to resolve. Since I am the one downloading the zip file and this functionality isn't user-facing, it's good enough for my use case but I wanted to post the patch so someone with a better understanding of StreamedResponses can pick up from here and maybe finish the last bit to get this working proper enough to be worthy of committing.

Steps to reproduce

Apply the supplied patch.
Go to /admin/content, check a few entities and perform the action "Download PDF separately".
Go to public://entity_print/print.zip for the output.

Proposed resolution

Add additional action to print entities separately, zip them and serve the zip file.

Remaining tasks

Serve the zip as a StreamedResponse and fix the deprecated error.
Add test case.

User interface changes

Adds an action called "Download PDF separately" to the content overview page.

API changes

n/a

Data model changes

n/a