Alerting: Add abstraction layer and testing hooks in front of SMTP dialer (#43875)

* Add abstraction layer above SMTP communication

* Fix issues with attachments and sync command

* Tests for bad SMTP behavior

* Separate tests between async and sync entry points. Test difference between them

* Return interface so Wire can properly map types

* Address feedback from George

pkg/services/notifications/testing.go

@@ -0,0 +1,32 @@
+package notifications
+
+import "fmt"
+
+type FakeMailer struct {
+	Sent []*Message
+}
+
+func NewFakeMailer() *FakeMailer {
+	return &FakeMailer{
+		Sent: make([]*Message, 0),
+	}
+}
+
+func (fm *FakeMailer) Send(messages ...*Message) (int, error) {
+	sentEmailsCount := 0
+	for _, msg := range messages {
+		fm.Sent = append(fm.Sent, msg)
+		sentEmailsCount++
+	}
+	return sentEmailsCount, nil
+}
+
+type FakeDisconnectedMailer struct{}
+
+func NewFakeDisconnectedMailer() *FakeDisconnectedMailer {
+	return &FakeDisconnectedMailer{}
+}
+
+func (fdm *FakeDisconnectedMailer) Send(messages ...*Message) (int, error) {
+	return 0, fmt.Errorf("connect: connection refused")
+}