Provide API to be notified when an intent handler is installed

[danielwiehl]

Is your feature request related to a problem? Please describe.

We have a button that issues an intent when clicked. We want to disable this button while no intent handler is installed. In version 1.0.0-rc.13 this is only possible for messages sent to a topic, but not for intents.

We know that we could block startup until all activators report readiness. However, we plan to disable this feature because many activators are slowing down the application startup.

Describe the solution you'd like

Similar to MessageClient#subscriberCount$, the IntentClient provides API to observe the subscriptions for an intent, taking into account the intentions of the requesting application.