Alerting: Support tls config for webhook receiver (#93513)

Adds the ability to configure tls settings on the webhook receiver (e.g. to skip server certificate validation)

pkg/services/notifications/testing.go

@@ -33,11 +33,3 @@ func NewFakeDisconnectedMailer() *FakeDisconnectedMailer {
 func (fdm *FakeDisconnectedMailer) Send(ctx context.Context, messages ...*Message) (int, error) {
 	return 0, fmt.Errorf("connect: connection refused")
 }
-
-// NetClient is used to export original in test.
-var NetClient = &netClient
-
-// SetWebhookClient is used to mock in test.
-func SetWebhookClient(client WebhookClient) {
-	netClient = client
-}