feat(coderd): add new dispatch logic for coder inbox (#16764)

This PR is [resolving the dispatch part of Coder
Inbocx](https://github.com/coder/internal/issues/403).

Since the DB layer has been merged - we now want to insert notifications
into Coder Inbox in parallel of the other delivery target.

To do so, we push two messages instead of one using the `Enqueue`
method.

coderd/notifications/types/payload.go

@@ -1,5 +1,7 @@
 package types
 
+import "github.com/google/uuid"
+
 // MessagePayload describes the JSON payload to be stored alongside the notification message, which specifies all of its
 // metadata, labels, and routing information.
 //
@@ -18,4 +20,5 @@ type MessagePayload struct {
 	Actions []TemplateAction  `json:"actions"`
 	Labels  map[string]string `json:"labels"`
 	Data    map[string]any    `json:"data"`
+	Targets []uuid.UUID       `json:"targets"`
 }