Alerting: Add EmbeddedContents as alternative embedding in smtp (#99937)

Adds support for embedding []byte in SmtpClient instead of filenames. This is backwards compatible as it uses a new field EmbeddedContents to add an alternative to the existing EmbeddedFiles which takes filenames.

pkg/services/notifications/notifications_test.go

@@ -132,6 +132,31 @@ func TestSendEmailSync(t *testing.T) {
 		require.Equal(t, []byte("text file content"), file.Content)
 	})
 
+	t.Run("When embedding readers to emails", func(t *testing.T) {
+		ns, mailer := createSut(t, bus)
+		cmd := &SendEmailCommandSync{
+			SendEmailCommand: SendEmailCommand{
+				Subject:     "subject",
+				To:          []string{"asdf@grafana.com"},
+				SingleEmail: true,
+				Template:    "welcome_on_signup",
+				EmbeddedContents: []EmbeddedContent{
+					{Name: "embed.jpg", Content: []byte("image content")},
+				},
+			},
+		}
+
+		err := ns.SendEmailCommandHandlerSync(context.Background(), cmd)
+		require.NoError(t, err)
+
+		require.NotEmpty(t, mailer.Sent)
+		sent := mailer.Sent[len(mailer.Sent)-1]
+		require.Len(t, sent.EmbeddedContents, 1)
+		f := sent.EmbeddedContents[0]
+		require.Equal(t, "embed.jpg", f.Name)
+		require.Equal(t, "image content", string(f.Content))
+	})
+
 	t.Run("When SMTP disabled in configuration", func(t *testing.T) {
 		cfg := createSmtpConfig()
 		cfg.Smtp.Enabled = false