[Synthetics] Support n-location check for monitor rules

[justinkambic]

Describe the feature:

Synthetics should contain a rule definition that allows users to only fire alerts when >= n locations are simultaneously down.

Describe a specific use case for the feature:

I am monitoring my website from London, Japan, US East, and Berlin.

I'm only interested in my website's overall availability, so being down in 1 location is not of interest to me. I only want to be alerted when my website is registering as down in 2 or more locations. When down in >= 2 locations, my rule should fire an alert.

[elasticmachine]

Pinging @elastic/uptime (Team:uptime)

[btosello]

This will be nice to have. In other tools as site24x7 they have this approach.

[shahzad31]

fixed by #186585