Add "override" config source

[tombruijn]

As discussed in https://github.com/appsignal/integration-guide/pull/128, we want to track some config options in the "override" source.

TODO

• Add "override" config source to the Config module
• Move the backwards compatability logic to be set through the "override" source, rather than after the fact as is done now
  • Move the overrides that are now reported in the "system" to the "override" source
  • Check if this actually applies to any config options for Node.js
• Update the diagnose report to report the "override" source
• Update Node.js load order docs page to mention the "override" source https://docs.appsignal.com/nodejs/configuration/load-order.html