smtp: add --ehlo-domain/--helo-domain to set the client greeting name

[codyprime]

The EHLO/HELO argument identifies the connecting client (RFC 5321 4.1.4), but the module sent the target's domain. These flags were removed in 589d4e0. Re-add flags to set it explicitly; when unset, behavior is unchanged (falls back to the target domain).

How to Test

New flag

echo mail.gmx.com \
  | go run ./cmd/zgrab2 smtp --blocklist-file=/dev/null --port=587 --ehlo-domain=example.com \
  | jq -r '.data.smtp.result.ehlo'

250-gmx.net Hello example.com [87.249.134.6]
250-8BITMIME
250-SIZE 141557760
250 STARTTLS

Regression (default behavior)

echo mail.gmx.com \
  | go run ./cmd/zgrab2 smtp --blocklist-file=/dev/null --port=587 --ehlo-domain=example.com \
  | jq -r '.data.smtp.result.ehlo'

250-gmx.net Hello mail.gmx.com [87.249.134.6]
250-8BITMIME
250-SIZE 141557760
250 STARTTLS