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/defaults/main.yml

@@ -1,3 +1,4 @@
 smtp_inet_interfaces: localhost
 smtp:
   mode: direct
+postfix_recipient_canonical_map: {}