private <C extends Builder<T>, T> List<T> append(C[] defaults, C[] overrides) {
        List<T> list = new LinkedList<T>();
        if (defaults != null) {
            for (C builder : defaults) {
                list.add(builder.build());
            }
        }
        if (overrides != null) {
            for (C builder : overrides) {
                list.add(builder.build());
            }
        }
        return list;
    }
Returns an immutable map by merging the override signer configuration into the default signer
configuration for the given theme.

@param defaults
           default signer configuration
@param override
           signer configurations overrides
@param theme
           used for message logging. eg region, service, region+service
aws-java-sdk-core/src/main/java/com/amazonaws/internal/config/InternalConfig.java