No system message is returned when a queued message is processed

[omidmogasemi]

Problem

When a queued message is processed in the SDK, there is no system message returned indicating the queued message was processed.

This leaves clients unable to know if and when a queued message was received by the model.

Proposed Solution

Return a system message indicating that a queued message was received by the model. This could be a new type, something like this appended to the SDKMessage type union:

export declare type SDKMessageReceived = {
    type: 'system';
    subtype: 'message_received';
    uuid: UUID;
};

[jacksondc]

This would be super helpful for Conductor