r/postfix: Support rewriting recipient addresses

The *postfix* role will now generate configuration and a lookup table
for [canonical address mapping][0] of email recipients.  To configure
the mapping, the `postfix_recipient_canonical_map` must be a dictionary
of source-target addresses, e.g.:

```yaml
postfix_recipient_canonical_map:
  my.bad.email@fake.test: my.real.email@example.com
```

[0]: https://www.postfix.org/ADDRESS_REWRITING_README.html#canonical

roles/postfix/templates/main.cf.j2

@@ -434,6 +434,10 @@ alias_maps = hash:/etc/aliases
 #alias_database = dbm:/etc/mail/aliases
 alias_database = hash:/etc/aliases
 #alias_database = hash:/etc/aliases, hash:/opt/majordomo/aliases
+{% if postfix_recipient_canonical_map %}
+
+recipient_canonical_maps = hash:/etc/postfix/recipient_canonical
+{% endif %}
 
 # ADDRESS EXTENSIONS (e.g., user+foo)
 #