feat(nextcloud): add support of imaginary an externalPreviewProvider

[wrenix]

Description of the change

Benefits

• faster image rendering for preview
• nextcloud container is not killed (e.g. OOM) if big and much preview images are generated

Possible drawbacks

Works for me.

TODO:

• write CI test

Checklist

• I have read the CONTRIBUTING.md doc.
• DCO has been signed off on the commit.
• Chart version bumped in Chart.yaml according to semver.
• (optional) Parameters are documented in the README.md

[wrenix]

ping @jessebot / @provokateurin do you like to review?

[provokateurin]

Sorry, I don't have the time for that atm. I hope I can do it at some point, but only at the beginning of next month the earliest.

[jessebot]

oh this is neat! May I ask what the difference between this and the preview generator app are? (I'm not against it btw, just want to learn more) I will review this more thoroughly as soon as I have a moment 🙏

[jessebot]

Looks good to me. Thanks for this! Perhaps we can add another PR for adding a CI test of some sort in the future?

[SIMULATAN]

Thank you for working on this 🚀
Just wanted to point out that, with Nextcloud 30, the OC\Preview\Imaginary provider no longer generates PDF previews. It may be worth considering adding the new ImaginaryPDF provider to the Imaginary notes in the README.md file.
See https://docs.nextcloud.com/server/latest/admin_manual/release_notes/upgrade_to_30.html#previews-for-pdf-files-with-imaginary
Thanks again, looking forward to using it to speed up my (terribly slow) Nextcloud media previewing ❤️

[wrenix]

rebased on main and add docu in e393d90 like @SIMULATAN suggested