Manage custom fields for Geocoding through plugins

[dkastl]

When Geocoding is enabled it requires certain custom fields to be created and these fields are linked through their name. This is at best a quick hack.

Instead we should manage the custom fields, if needed for Geocoding, through the plugin.
If these fields are only needed for certain Geocoding providers/use cases, they should be configurable in plugin settings.

It should not be necessary to manually create custom fields. The fields should not be referenced by name (label).