add TransportDirect transport

The `TransportDirect` transport allows to tweak outgoing SMTP
connections to remote servers. Currently, it only allows to select
network IP family (ipv4, ipv6 or both).

For example, to disable ipv6 for all outgoing SMTP connections:
- add these lines in mox.conf to create a new transport named
"disableipv6":
```
Transports:
  disableipv6:
    Direct:
      DisableIpv6: true
```
- then add these lines in domains.conf to use this transport:
```
Routes:
  -
    Transport: disableipv6
```

fix #149

config/doc.go

@@ -638,6 +638,18 @@ See https://pkg.go.dev/github.com/mjl-/sconf for details.
 				# typically the hostname of the host in the Address field.
 				RemoteHostname:
 
+			# Like regular direct delivery, but allows to tweak outgoing connections.
+			# (optional)
+			Direct:
+
+				# If set, outgoing SMTP connections will *NOT* use IPv4 addresses to connect to
+				# remote SMTP servers. (optional)
+				DisableIPv4: false
+
+				# If set, outgoing SMTP connections will *NOT* use IPv6 addresses to connect to
+				# remote SMTP servers. (optional)
+				DisableIPv6: false
+
 	# Do not send DMARC reports (aggregate only). By default, aggregate reports on
 	# DMARC evaluations are sent to domains if their DMARC policy requests them.
 	# Reports are sent at whole hours, with a minimum of 1 hour and maximum of 24