Allows the forward plugin to execute the next plugin based on the return code. Similar to the externally mainted alternate plugin https://github.com/coredns/alternate
Based on the idea of chrisohaver@ in #6549 (comment)
Also incoperated the request to rename `alternate` to `next` as an option
I am having issues adding a proper test for functionality. Primarily, I do not know the code base enough and having multiple `dnstest.NewServer` with ResponseWriter does not work. From my testing these are "Singletons'' and only the last defined response writer is used for all servers
Signed-off-by: Jasper Bernhardt <jasper.bernhardt@live.de>