[Synthetics] Improve synthetics alerting

[shahzad31]

Summary

Fixes https://github.com/elastic/kibana/issues/175298

Improve synthetics alerting !!

User will be able to create custom synthetics status alert by defining three kind of criteria

Monitor is down over last consective checks with threshold

From Locations threshold

Will be considered down only when from defined number of locations

Over time with checks threshold just like uptime custom status alert