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/dump.sql

@@ -113,7 +113,8 @@ CREATE TYPE notification_message_status AS ENUM (
 
 CREATE TYPE notification_method AS ENUM (
     'smtp',
-    'webhook'
+    'webhook',
+    'inbox'
 );
 
 CREATE TYPE notification_template_kind AS ENUM (