feat: PDF Template Workflow

[elzody]

• Resolves: #3711
• Target version: main

Summary

API which handles the extraction and filling of form controls in template documents. This means that a user will be able to input form controls in the document via Collabora Online, and when creating a new document based on that template, have the option to fill out the fields and they will be populated when the document is created. This is required for the workflow and automation improvements.

Related server pull request: nextcloud/server#46383

TODO

• [x] Also accept any document, not just templates -- user may want to get/fill fields from other documents as well
• [ ] Testing, would need help with this, was not working locally when I tried :)

Checklist

• [x] Code is properly formatted
• [x] Sign-off message is added to all commits
• [ ] Documentation (manuals or wiki) has been updated or is not required

[juliushaertl]

Just noticed, the field classes seem to be missing from the commits ;)