Alerting: Adds support for sending a single email to all recipients in notification channel (#21091)

When an alert is sent by e-mail, the process sends an e-mail to 
each recipient separately. This PR is a single delivery to all recipients.
For companies that use e-mail extensively, this is necessary in order 
not to overload the sending queue.

Replaces #18013
Fixes #12650

Co-authored-by: Henrique Oliveira <holiiveira@users.noreply.github.com>

pkg/models/notifications.go

@@ -14,6 +14,7 @@ type SendEmailAttachFile struct {
 // SendEmailCommand is command for sending emails
 type SendEmailCommand struct {
 	To            []string
+	SingleEmail   bool
 	Template      string
 	Subject       string
 	Data          map[string]interface{}