Allow filtering on ICMP and optionally specify message type in `NetworkPolicy`

# Summary

Currently, we only allow filtering by `TCP`, `UDP` and `SCTP` (`SCTP` also being highly irrelevant).
We should also allow `ICMP` as protocol and, when specified, allow filtering by ICMP message type.

## Basic example

```yaml
protocol: ICMP
messageTypes: [EchoReply]
```