[Feature] [Application Gateway for Containers] Add support for a default network policy to Helm chart

**Is your feature request related to a problem? Please describe.**
Would it be possible to implement a base supported network policy in the helm chart?

Optional: Set the appropriate GitHub label(s) to indicate the AKS feature or area your feature request relates to.
app-gateway-for-containers

**Describe the solution you'd like**
Having a a networkPolicy block in helm chart, I'm guessing something like

```
networkPolicy:
enabled: false
ingres: []
egress:
- ports:
- port: 443
protocol: TCP
- port: 53
protocol: TCP
- port: 53
protocol: UDP
to:
- ipBlock:
cidr: 0.0.0.0/0
- ipBlock:
cidr: "::/0"
```
with some default rules, I assume outbound on tcp/443 ad the main one, and then inbound 8000/8001 from some services.

**Describe alternatives you've considered**
I know I can deploy this on the side, but would be nice to also have it easy deployable with the Helm chart.

**Additional context**
N/A