feat: add level support for startup logs (#7067)

This allows external services like our devcontainer support to display
errors and warnings with custom styles to indicate failures to users.

coderd/database/dump.sql

@@ -501,7 +501,8 @@ CREATE TABLE workspace_agent_startup_logs (
     agent_id uuid NOT NULL,
     created_at timestamp with time zone NOT NULL,
     output character varying(1024) NOT NULL,
-    id bigint NOT NULL
+    id bigint NOT NULL,
+    level log_level DEFAULT 'info'::log_level NOT NULL
 );
 
 CREATE SEQUENCE workspace_agent_startup_logs_id_seq