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/migrations/000116_startup_logs_level.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE workspace_agent_startup_logs
+	DROP COLUMN level;

coderd/database/migrations/000116_startup_logs_level.up.sql

@@ -0,0 +1,2 @@
+ALTER TABLE workspace_agent_startup_logs
+	ADD COLUMN level log_level NOT NULL DEFAULT 'info'::log_level;