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/secret.yaml

@@ -0,0 +1,7 @@
+apiVersion: v1
+data:
+  token: KipTT1BTKio=
+kind: Secret
+metadata:
+  name: secret_ignore
+type: Opaque