[Feature request] Notification on subset of events (only UP/DOWN)

**Is your feature request related to a problem? Please describe.**
I would like to receive only up or down events of a monitor.
I have a monitor, that if it turns down, I trigger a webhook that does some actions

**Describe the solution you'd like**
I would like to set a flag or a checklist, ideally per notification provider, that allows me to receive only a subset of events.

**Describe alternatives you've considered**
none

**Additional context**
none

Look at the notification provider configuration and event handling code. Identify where monitor status changes (UP/DOWN) are processed and where notifications are dispatched. The change likely involves adding a filter per provider to only send notifications for selected event types. Check existing tests for notification logic to understand the expected behavior.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.