KEP-1880 Multiple Service CIDRs (kubernetes#3365)

* KEP-1880 Multiple Service CIDRs

Kubernetes networking is based fundamentally on two kind of networks,
the Pod network aka Cluster Network and the Service Network. These
networks are configured statically, using flags, causing a lot of
limitations and pain on users and administrators.

The Pod network is consumed by the CNI plugins, Kubernetes already provides
a simple IPAM that will be extended by the KEP-2593 "Multiple Cluster
CIDRs", allowing users to use an API to configure it.

This KEP provides a similar API for the Service Network, allowing users
to dynamically add more networks or resize the existing ones. It also
remove some of the limitations of current implementation.

* clarify serviceCIDRConfig IPAddress relation

keps/prod-readiness/sig-network/1880.yaml

@@ -0,0 +1,3 @@
+kep-number: 1880
+alpha:
+  approver: "@johnbelamaric"