Support (or allow) HELP command implementation

Currently conn.go does not support HELP SMTP command and throws a 502.

This presents a problem because many clients, like curl, request HELP upon connection.

```
curl -v "smtp://$SMTP_SERVER_HOST:$SMTP_SERVER_PORT"
* Trying 34.212.234.28:2525...
* Connected to redacted (REDACTED) port 2525
< 220 redacted ESMTP Service Ready
> EHLO Jacks-MBP-3
< 250-Hello Jacks-MBP-3
< 250-PIPELINING
< 250-8BITMIME
< 250-ENHANCEDSTATUSCODES
< 250-CHUNKING
< 250-STARTTLS
< 250-AUTH PLAIN
< 250 SIZE 104857600
> HELP
< 502 5.5.1 HELP command not implemented
* Command failed: 502
> QUIT
< 221 2.0.0 Bye
* Closing connection
curl: (8) Command failed: 502
```

Do you have any plans to implement or allow implementation (by interface extension) of basic HELP response? I really admire and appreciate your work. Happy to add PR if this is acceptable.

Start in conn.go and reproduce the SMTP session with the curl command shown in the issue, focusing on how unsupported commands return 502. Determine the project’s existing server command-handling extension points, then make HELP produce an accepted basic response or be implementable through the proposed interface; done means curl no longer fails on HELP.

Written by the indexing model from the issue text.