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/database/migrations/000299_notifications_method_inbox.down.sql

@@ -0,0 +1,3 @@
+-- The migration is about an enum value change
+-- As we can not remove a value from an enum, we can let the down migration empty
+-- In order to avoid any failure, we use ADD VALUE IF NOT EXISTS to add the value

coderd/database/migrations/000299_notifications_method_inbox.up.sql

@@ -0,0 +1 @@
+ALTER TYPE notification_method ADD VALUE IF NOT EXISTS 'inbox';