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/server/wire.go

@@ -150,6 +150,7 @@ var wireBasicSet = wire.NewSet(
 	libraryelements.ProvideService,
 	wire.Bind(new(libraryelements.Service), new(*libraryelements.LibraryElementService)),
 	notifications.ProvideService,
+	notifications.ProvideSmtpService,
 	tracing.ProvideService,
 	metrics.ProvideService,
 	testdatasource.ProvideService,