[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.

由索引模型根据 Issue 内容生成。