Add SMTP_HELLO_NAME to let operators set the EHLO/HELO identity

Google Workspace SMTP relay (smtp-relay.gmail.com) rejects generic
EHLO identifiers such as "localhost", which is Go net/smtp's default.
Operators can now set SMTP_HELLO_NAME to a valid hostname so that
c.Hello() is called before StartTLS, satisfying strict relay policies.

Fixes https://github.com/getprobo/probo/issues/1284

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/probodconfig/mailer_config.go

@@ -26,4 +26,5 @@ type SMTPConfig struct {
 	User        string `json:"user"`
 	Password    string `json:"password"`
 	TLSRequired bool   `json:"tls-required"`
+	HelloName   string `json:"hello-name"`
 }