Add flag enable-disk

What does this MR do?

• Adds enable-disk flag -> allows to explicitly enable disk, on by default but will turn into off by default from %14.0
• When enable-disk=true -> serve from either source.Type = file or zip, independently from API response (effectively no change from current behaviour)
• When enable-disk=false -> serve 500 if source.Type = file or source.Path.HasPrefix("file://")
• This change is completely independent from -domain-config-source

TODO

• Added the Changelog (e.g. Changelog: feature) trailer to the commits that need to be included in the changelog
• Added unit tests (if required)
• Added acceptance tests (if required)
• Documentation (if required)
• Code review guidelines
• Go Style guidelines

Closes #543 (closed)