Support Azure Activity Log alerts

[dougbu]

The current Azure Alert receivers (ASP.NET and ASP.NET Core) support Azure Metric alerts and (modulo #146) Availability / Web Test alerts. However Activity Log alerts use a completely different JSON schema, one that does not contain the required context:name field.

To support Activity Log alerts, update how the Azure Alert receivers find event names. One option is to search the schema for something like an event name (see https://docs.microsoft.com/en-us/azure/monitoring-and-diagnostics/monitoring-activity-log-alerts-webhook and linked details e.g. https://docs.microsoft.com/en-us/azure/monitoring-and-diagnostics/monitoring-service-notifications). Another option is to fall the event name back to a constant value or to allow the event name to be null.

Secondarily, should add strong types per Activity Log alert types e.g. one for Administration alerts and another for Service Health alerts.

[dougbu]

https://docs.microsoft.com/en-us/azure/monitoring-and-diagnostics/monitoring-activity-log-schema contains more details about the various Activity Log alerts.

[aspnet-hello]

This issue was copied to aspnet/AspNetWebHooks#22

[aspnet-hello]

This issue was moved to aspnet/AspLabs#47