Allow per-monitor email addresses

[wortmanb]

Is your feature request related to a problem? Please describe.
My customer uses Synthetic Monitors to keep uptime records for various hosted services, and when a service goes down for a time, they'd like a way to have the responsible team emailed directly. Currently, the only way to set emails is a default for all synthetic-based alerts.

Describe the solution you'd like
Each monitor should include connectors and emails to allow per-monitor overrides of the defaults so individuals or teams can receive immediate notification of monitors they are interested in instead of having to be subscribed to all monitors.

[smith]

This probably should be moved to https://github.com/elastic/synthetics-dev/issues.

[wortmanb]

Do I need to move it?