Allow syncing to be disabled

**Describe the problem/challenge you have**

I would like to be able to disable syncing entirely for an App and only have that App updated by explicit request from a client.

**Describe the solution you'd like**

On the App spec, there should be some way to disable syncing for an App and then reconciliation only happens via a client request to kapp-controller.

```yaml
apiVersion: kappctrl.k14s.io/v1alpha1
kind: App
metadata:
name: simple-app
namespace: default
spec:
disableSyncPeriod: true
serviceAccountName: default-ns-sa
fetch:
- git:
url: https://github.com/k14s/k8s-simple-app-example
ref: origin/develop
subPath: config-step-2-template
template:
- ytt:
inline:
pathsFrom:
- secretRef:
name: simple-app-values
deploy:
- kapp: {}
```