unbound: block list source selection support for community edition #9136

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• I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
• I am convinced that my issue is new after having checked both open and closed issues at https://github.com/opnsense/core/issues?q=is%3Aissue

Is your feature request related to a problem? Please describe.

Source selection is a business edition feature, but we want to offer a similar functionality for the community edition as well now.

Describe the solution you like

Add a GUI using the same available pluggable framework, but likely should not interfere with the business feature.

Describe alternatives you considered

Use an external solution for this, which complicates management. Use the business edition, which is not an option for some users.

Additional context

N/A