Add UserContent mechanism

[timmjd]

What this PR does / why we need it

Implements a way to provide User Content files that will be served by Jenkins. Common use-case is to let Jenkins host a svg or png together with a custom css to customize the UI.

Which issue this PR fixes

Special notes for your reviewer

Checklist

• [x] DCO signed
• [x] Chart Version bumped
• [x] CHANGELOG.md was updated

[timmjd]

The use-case I'd like to solve is to let Jenkins host a simple svg or png together with a custom css to adapt the styling of the UI - and that seems to be a quiet common use-case:

• https://medium.com/@elhayefrat/replace-jenkins-logo-and-text-to-your-choice-in-jenkinsui-f7d35daed25b
• https://nadavsvirsky.medium.com/customize-jenkins-favicon-dashboard-logo-f5c0b70d817d
• https://stackoverflow.com/a/70360371

To achieve this with the current implementation:

• Use the persistence.volumes & persistence.mounts option
  • Would require an external ConfigMap / Secret / PVC to be created outside the Helm context
• Create a custom Docker image where the content gets baked in
  • Would require to build & host the docker image: Additional CI & Hosting infrastructure
• Download the resources from an external source
  • Requires to host the content via a webserver infrastructure
• Write a plugin to host the content
  • Total overkill

All of these come with more or less overhead. Therefore, letting Jenkins host the files itself would be the most elegant one. Not sure if there is a more easy way to achieve this - any hint would be great.

[timmjd]

Hey @torstenwalter, I refactored to work with ConfigMaps instead of Secrets. Also it's possible to provide UTF-8 or binary encoded content separately. What's your suggestion?

[torstenwalter]

@timmjd Sorry I lost track of this. You can ping me once the merge conflicts are resolved and a test is added otherwise this looks good to me.