Adding APIs for finer control over a WorkflowHost's renderings

[johnnewman-square]

This PR adds a managedRenderings property to WorkflowHost, behind a WorkflowHostManagement SPI. When this property is enabled, shouldSkipRenderForOutput(_:) returns true so that the host's workflow isn't rendered. A new managedUpdate(workflow:) API can be used to perform the actual rendering.

The planned use case for this API is when a container attaches one or more WorkflowHost instances to a workflow tree, bridging the host's renderings and output into the main tree and rerendering the host when the main tree renders. Without this API, the attached host's workflow will render an extra time when actions are applied: once when the action is applied and once when the main tree renders. This API allows the container to render the hosted workflow only after the main tree renders.

Checklist

• Unit Tests
• UI Tests
• Snapshot Tests (iOS only)
• I have made corresponding changes to the documentation