feat(lib/policy/expressions): add system load average to bot expression inputs

This lets Anubis dynamically react to system load in order to
increase and decrease the required level of scrutiny. High load? More
scrutiny required. Low load? Less scrutiny required.

lib/policy/celchecker.go

@@ -73,6 +73,12 @@ func (cr *CELRequest) ResolveName(name string) (any, bool) {
 		return expressions.URLValues{Values: cr.URL.Query()}, true
 	case "headers":
 		return expressions.HTTPHeaders{Header: cr.Header}, true
+	case "load_1m":
+		return expressions.Load1(), true
+	case "load_5m":
+		return expressions.Load5(), true
+	case "load_15m":
+		return expressions.Load15(), true
 	default:
 		return nil, false
 	}