Support internal Load Balancing for Kubernetes stacks

On the server v0.4.21 has introduced a better way of dealing with
intra-stack networking: if the user can specify a list of endpoints
exposed internally, we now can setup a ClusterIP for this to avoid the
pitfalls of DNS-based load balancing.
This exposes the feature using the "Expose" compose field, and adds an
extra x-internal-service-type field to explicitly define how intra-stack
networking is handled on a service.

Signed-off-by: Simon Ferquel <simon.ferquel@docker.com>

cli/command/stack/kubernetes/testdata/compose-with-expose.yml

@@ -0,0 +1,9 @@
+version: "3.7"
+services:
+  test:
+    image: "some-image"
+    expose:
+    - "1" # default protocol, single port
+    - "2-4" # default protocol, port range
+    - "5/udp" # specific protocol, single port
+    - "6-8/udp" # specific protocol, port range