Allow per-monitor email addresses

**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.

The issue names no files, tests, or entry points. Start by locating synthetic monitor alert configuration and the existing default email or connector handling, then determine how monitor-level values should override those defaults. Done means individual monitors can notify their own responsible teams without subscribing them to every monitor.

Written by the indexing model from the issue text.