Add support for configuring PF

In the example we have the syntax:

```
[network.net3.tap]
bridge = "bridge2"
ip = "192.168.100.1/24"
nat = true
nat_if = "en0"
pf_rules = [
  "nat on en0 inet from 192.168.101.0/24 to any -> (en0)"
]
```

The NAT and pf_rules functionality needs to be implemented.