LTI Platform Notification Service

The Platform Notification Service adds a third messaging channel to LTI to facilitate Platform-to-Tool messages that can occur outside the boundaries of an active user agent session. After discovering this service in the standard way, the Tool can use a service request to: Discover the various platform *notice types* the Platform supports (So that the Tool may register to receive *notices* of those types). Manage the registration of a Tool-hosted HTTP *webhook* or *handler* endpoint with the Platform to receive these notices.