Add the possibility to ignore files with build and diff ks

If implemented, user will be able to ignore files when using `build
kustomization` and `diff kustomization` both with .sourceignore and
`ignore-paths` flag.

Signed-off-by: Soule BA <bah.soule@gmail.com>

cmd/flux/testdata/build-kustomization/ignore/not_deployable/ignore_svc.yaml

@@ -0,0 +1,17 @@
+apiVersion: v1
+kind: Service
+metadata:
+  name: do_not_deploy
+spec:
+  type: ClusterIP
+  selector:
+    app: podinfo
+  ports:
+    - name: http
+      port: 9898
+      protocol: TCP
+      targetPort: http
+    - port: 9999
+      targetPort: grpc
+      protocol: TCP
+      name: grpc