Provide an Azure Event Grid event source with ListenerConnected and ListenerDisconnected events

# Description
With the [Hybrid Connections - Get](https://docs.microsoft.com/en-us/rest/api/relay/hybrid-connections/get?tabs=HTTP) API, we can easily retrieve the number of currently connected listeners of a Hybrid Connection.

It would be great if we could get `ListenerConnected` and `ListenerDisconnected` events in Azure Event Grid for this, such that our application can respond to listeners coming up or going down. For example, it would be great if we could show in our application's UI that a listener is currently connected, and have that status display change when the listener disconnects, without polling Azure's REST API all the time.

## Actual Behavior
The [Azure Service Bus as an Event Grid source](https://docs.microsoft.com/en-us/azure/event-grid/event-schema-service-bus) article shows that there are currently four events exposed from Azure Service Bus via Azure Event Grid, but none of them related to Hybrid Connections. Also, the [Event Grid overview page](https://docs.microsoft.com/en-us/azure/event-grid/overview) does not list Azure Relay as a dedicated technology that exposes events via Event Grid.

## Expected Behavior
`ListenerConnected` and `ListenerDisconnected` events should be available via Azure Event Grid, either via a dedicated Azure Relay event source, or via the existing Azure Service Bus event source.