Add support for more Azure FunctionApp triggers.

[CyrusNajmabadi]

We currently support http/topic/container/queue triggers. However, now that we're on azure functions 2.0 we can support the following as well:

1. EventGrid triggers
2. EventHub triggers
3. Timer triggers

[mikhailshilkov]

4. Cosmos DB trigger

[mikhailshilkov]

So, the Event Grid trigger implementation is a bit more tricky:

1. In order to link an Event Grid subscription to an Azure Function, the webhook URL must include a system key.

2. There is an HTTP API to get a system key, but it requires a master key.

3. None of the keys are available from Pulumi resources.

4. None of the keys are available from Terraform resources: issue.

5. It's not there because Azure Go SDK doesn't support it.

6. The last comment there mentions that there will be a new API available soon from the Functions side. Here is the issue to track. The new API on Functions is now deployed.

There is an alternative path: create an HTTP-triggered function and handle EventGrid webhook in manual mode: respond to validation and parse the event from HTTP request.

The downside is that we get an open HTTP function hanging around. The upside: flexibility to get events in Cloud Events standard shape.

[mikhailshilkov]

I'm watching Azure/azure-functions-host#3994 which should simplify the implementation of the Event Grid trigger (no need to call Kudu APIs)

[mikhailshilkov]

Everything here except EventGrid is done. Closing in favor of #295.