[MM-48528] Add support for SendGrid X-SMTPAPI header in email sends (#22296)

* Add support for SendGrid X-SMTPAPI header for categorizing sent emails to track performance

* Implement feedback

* Remove unnecessary struct

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

app/admin.go

@@ -195,7 +195,7 @@ func (a *App) TestEmail(userID string, cfg *model.Config) *model.AppError {
 	T := i18n.GetUserTranslations(user.Locale)
 	license := a.Srv().License()
 	mailConfig := a.Srv().MailServiceConfig()
-	if err := mail.SendMailUsingConfig(user.Email, T("api.admin.test_email.subject"), T("api.admin.test_email.body"), mailConfig, license != nil && *license.Features.Compliance, "", "", "", ""); err != nil {
+	if err := mail.SendMailUsingConfig(user.Email, T("api.admin.test_email.subject"), T("api.admin.test_email.body"), mailConfig, license != nil && *license.Features.Compliance, "", "", "", "", ""); err != nil {
 		return model.NewAppError("testEmail", "app.admin.test_email.failure", map[string]any{"Error": err.Error()}, "", http.StatusInternalServerError)
 	}