Add resolver option similar to mailauth resolver option

[titanism]

Right now it's not possible to pass a custom resolver function to resolve hostname lookup.

smtp-server/lib/smtp-connection.js

Line 214 in 2bd0975

dns.reverse(this.remoteAddress.toString(), (...args) => {

The package mailauth supports a resolver option, and it would be helpful to have this across the entire smtp-server stack for DNS queries.

Thanks!

[titanism]

Also, you may want to catch the error code instead for dns.reverse errors thrown, e.g. https://github.com/omrilotan/reverse-dns-lookup/blob/main/index.js#L40.