Aggiunge notifier server per scheduler

tests/test_server_notifier.py

@@ -0,0 +1,18 @@
+from __future__ import annotations
+
+from bakrest.server_check import ServerStatus
+from bakrest.server_notifier import should_alert
+
+
+def test_notifier_alerts_once_while_server_is_down() -> None:
+    status = ServerStatus(False, "now", "down")
+
+    assert should_alert(status, None)
+    assert not should_alert(status, "unreachable")
+    assert should_alert(status, "unreachable", always_alert=True)
+
+
+def test_notifier_does_not_alert_when_server_is_reachable() -> None:
+    status = ServerStatus(True, "now", "ok")
+
+    assert not should_alert(status, None)