Make protocol optional in config

[edwardwc]

Ex config:

# API reference for the YAML fields

# The interface to run the XDP on
interface: "enp43s0np0"
# Whether the firewall is persistent across reboots.
# Not currently implemented.
# persistent: true

# Whether the Prometheus exporter is enabled.
# Not currently implemented. Prometheus always listens on :4343.
# prometheus: true
# The local port Prometheus should listen on.
# prometheus_port: 4242

# The firewall rules you'd like to define.
# The firewall drops traffic like TCP and UDP by default, rules whitelist traffic
rules:
  # Define each rule individually
  - rule:
    # The source IP range this rule will apply to. For example, 23.133.104.69/32, or 23.133.104.0/24.
    # To allow traffic from any IP, use 0.0.0.0/0
    source_ip_range: 5.161.57.23/32
    # The destination IP range this rule will apply to.
    # To allow traffic to go to any IP assigned with this server, use 0.0.0.0/0.
    destination_ip_range: 5.161.57.23/32
    # The IP protocol to allow.
    # Current allowed values are: TCP, UDP, ICMP, ALL.
    protocol: "TCP"
    # The port to allow the traffic to. Only applicable to TCP and UDP.
    # Omit or enter 0 to allow any port.
    port: 0
    # Limit the amount of packets sent to this service per source IP. Runs over 1 minute.
    # Enter to zero to disable ratelimiting.
    ratelimit: 0

protocol should be optional.