Regroup all kubernetes extra-fields under x-kubernetes

This regroup all Kubernetes extra fields for compose-on-kubernetes
v1alpha3 in a single x-kubernetes object.
Also use the same naming scheme as cap_add etc. for fiels inside this
object.

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

cli/command/stack/kubernetes/testdata/compose-with-pull-policy.yml

@@ -2,4 +2,5 @@ version: "3.7"
 services:
   test:
     image: "some-image"
-    x-pull-policy: "Never"
+    x-kubernetes:
+      pull_policy: "Never"

cli/command/stack/kubernetes/testdata/compose-with-pull-secret.yml

@@ -2,4 +2,5 @@ version: "3.7"
 services:
   test:
     image: "some-private-image"
-    x-pull-secret: "some-secret"
+    x-kubernetes:
+      pull_secret: "some-secret"