@SmithyInternalApi
    public static String getKeyString() {
        return LazyEndpointComponentFactoryHolder.INSTANCE.getKeyString();
    }
Gets the built-in names as a joined string.

@return a string of the built-in names.
smithy-rules-engine/src/main/java/software/amazon/smithy/rulesengine/language/EndpointRuleSet.java