[FEATURE REQUEST] Request (optional) permission to "install apps from unknown sources"

Is your feature request related to a problem? Please describe.
I wanted to securely share an APK with my phone to install it. When I tried to install it from the OwnCloud Android client, Android offered my to open the APK with the "package installer", but nothing happened. I also couldn't manually enable the "install apps from unknown sources" feature in the OwnCloud app's settings, as the app doesn't seem to request this feature (MANIFEST?).

I was unable to download the APK to my "Downloads" folder with OwnCloud, so that I could have used the Android "Files" app to install it.

Describe the solution you'd like
I would like for the OwnCloud app to (optionally) request this permission to give users the option to install APKs they have on their OwnCloud.

Describe alternatives you've considered
Alternative would be to use some other means to download the APK to your phone, but those may potentially not be as safe as using your own trusted OwnCloud.

Additional context
N/A

TASKS

• Research (if needed)
• Create branch feature/feature_name
• Development tasks
  • Implement whatever
  • ...
  • Implement unit tests (if needed)
• Code review and apply changes requested
• Design test plan
• QA
• Merge branch feature/feature_name into master

The issue names no files, tests, or entry points. Start by tracing how the Android client downloads and opens APKs, then check its manifest and current handling of Android's unknown-sources permission. Done means defining the optional permission behavior, supporting APK installation from OwnCloud when permitted, and documenting or testing the relevant Android flows.