[Synthetics] Allow configuring monitor downtime alert on no data

[vigneshshanmugam]

Describe the feature:

Synthetics alerting has been dramatically improved lately #186585 which gives the users ability to configure custom rules and allows filtering monitor by name, tags, locations etc.

It would be extremely useful to have an alert option for when a monitor is not running. Currently, alerts can only be set for when a monitor is down, but having the ability to trigger alerts when there’s no monitor status detected would help catch issues like misconfigurations or missed monitor runs. This would essentially serve as another way to detect downtime.

Right now, the workaround is to use a Custom Threshold rule to alert on no data, but a built-in option for this would greatly improve usability.

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)

[jasonrhodes]

@maryam-saeidi brought up in meeting that we have related issues around what it means for a rule to detect "no data" and what that should mean for the rule's state and its corresponding alerts. See conversation here: #165421

[jasonrhodes]

Spoke with @drewpost, we don't currently have capacity for this right now. Moving this to our backlog for now.