[Feature] external_relay: add ip_map strategy

[moesoha]

Add a strategy named ip_map for finding relays by IP/network map.

Example config:

rules {
  EXTERNAL_RELAY_HOST {
    strategy = "ip_map";
    ip_map = [
      "2001:db8::/32",
      "192.0.2.0/24"
    ];
  }
}

This PR should not be squashed as it contains commits for two goals.

[fatalbanana]

For testing it may be useful to set a lower priority (like 9?) on all rules in external_relay configuration; probably these tests are just barely hanging together as it is- sorry about it.

In general this LGTM.

[moesoha]

For testing it may be useful to set a lower priority (like 9?) on all rules in external_relay configuration; probably these tests are just barely hanging together as it is- sorry about it.

In general this LGTM.

Oh I also noticed this problem, adjusting rules.

[vstakhov]

Looks good in general with some minor nits. Thank you!

[vstakhov]

Should it be a error notice? I think it should be either debug or info maximum. Users generally don't like too verbose logs.

[moesoha]

everywhere in this module uses errx for logging such message. Of course I can change all such conditions if you want.

[vstakhov]

This logic is a bit unclear. What is the overall goal of the check i == num_rcvd? It seems that it might break the exisiting logic of passing smtp envelope data to Rspamd.

[moesoha]

I am wondering if a message is originated from a relay host, the real Received header decision will be wrong if we remove this.

[moesoha]

I added a lower priority to EXTERNAL_RELAY_COUNT to fix the test cases. This is in a separate commit so this PR should not be squashed.