enable preview providers via a boolean in the helm chart

Description of the change

enable preview providers via a boolean in the helm chart

Benefits

Many wish to install nextcloud via the helm chart without necessarily learning all the deeply technical inner workings of how nextcloud actually works behind the scene. Enabling thumbnails/previews is a common ask after installing and using the mobile app for the first time. At this point, revisiting the helm chart values.yaml and searching to enable the thumbnails/previews is the next step and this should be simple, and not require having to know how to modify the nextcloud config files.

Possible drawbacks

Implementing this could lead to other configuration options being set without requiring having to know the internal workings of nextcloud, but I think this would be a good thing, not a drawback. Though, just enabling a reverse-proxy and enabling thumbnails is all I've ever needed over the last 4 years. Making the configuring of OIDC also would be nice. ... but for this feature request just looking to enable previews

Additional

nextcloud:
  # takes a little extra cpu and is not therefore enabled by default
  preview:
    enable: false
    providers:
      txt: true
      image: true
      movie: true
      mp4: true

Start with the chart's values.yaml and the templates that currently configure Nextcloud settings. Trace how existing chart values reach the Nextcloud configuration, then determine how the proposed preview boolean and provider values should be represented. Done means preview support can be enabled through chart values without manually editing Nextcloud configuration.