Make it possible to disable Externalname support

[mflendrich]

Problem Statement

Per CVE-2021-25740, it is possible for ExternalName services to target services living in other namespaces. This is a consequence of the Kubernetes design.

It might be desirable to allow users to disable support for Externalname services to make cross-namespace traffic more difficult in scenarios where it is not desirable. Whether this should be considered a security issue is under debate.