[Event Grid] Investigate better typing of required system events properties

[ellismg]

Today we generate the shapes of system events using auto-rest based on the canonical definitions of the the events in https://github.com/Azure/azure-rest-api-specs/tree/master/specification/eventgrid/data-plane. Jeff notes that the generated code here doesn't have as strong a typing as we might want. For example, if you look at AppConfigurationKeyValueDeletedEventData, which corresponds to this scchema you'll note we generate a type like this:

// @public
export interface AppConfigurationKeyValueDeletedEventData {
    etag?: string;
    key?: string;
    label?: string;
}

It feels like the expectation here is that etag, key and label will always be set, yet our autogenerated interfaces have them as optional. We should figure out if these actually can be undefined in practice or if the service team ensures that all members are actually sent.

[ellismg]

In #10059 (comment), Jeff also points out places where we may want to use unknown instead of any, specifically for DeviceTelemetryEventProperties#body but there may be others.