Feature Request: Suppress sources in GetCapabilities

[citizenfish]

At present mapcache uses every source to complete a GetCapabilities request. We have some layers that require authentication (we have a auth proxy in front of mapcache), it would be good if a tag could be added to a source to prevent it being listed in GetCapabilities.

Eg:-

    <source>
      <getcapabilities_nolist>true</getcapabilities_nolist>
   ...