Create an external-dns controller that operates in the central control cluster.  It's job is to periodically instantiate external-dns jobs for each edge cluster.  The extneral-dns source will be DNS CRDs on the edge cluster, and the DNS provider will be whatever provider has been set up for that given edge cluster / project.  

The external-dns controller will need to query the DNS provider information (e.g. type, location, credentials, etc.) and instantiate an exteranl-dns job to do the work.