Misleading NetworkInterface configuration

Currently, [`NetworkInterface.StaticConfiguration.IPConfiguration.IPAddr`](https://pkg.go.dev/github.com/firecracker-microvm/firecracker-go-sdk#IPConfiguration) is of type `net.IPNet` which, according to the `net` library, represents

```go
// An IPNet represents an IP network.
type IPNet struct {
IP IP // network number
Mask IPMask // network mask
}
```

Note that there is no place to specify an actual IP address in there, only subnet and mask.
If the SDK user uses the convenient `net.ParseCIDR()` function, the resulting `net.IPNet` struct can be passed as-is to the firecracker config, but there is NO IP address that can be assigned to the VM's interface. This leads to the situation where the subnet ID (the network number) is assigned to the interface, which is very very wrong.

I believe a different type should be used for this setting.