feat(coderd): add mark-all-as-read endpoint for inbox notifications (#16976)

[Resolve this issue](https://github.com/coder/internal/issues/506)

Add a mark-all-as-read endpoint which is marking as read all
notifications that are not read for the authenticated user.
Also adds the DB logic.

coderd/database/queries/notificationsinbox.sql

@@ -57,3 +57,11 @@ SET
     read_at = $1
 WHERE
     id = $2;
+
+-- name: MarkAllInboxNotificationsAsRead :exec
+UPDATE
+	inbox_notifications
+SET
+	read_at = $1
+WHERE
+	user_id = $2 and read_at IS NULL;