public static <K, V, VO> Joined<K, V, VO> keySerde(final Serde<K> keySerde) {
        return new Joined<>(keySerde, null, null, null, null);
    }
Create an instance of {@code Joined} with  a key {@link Serde}.
{@code null} values are accepted and will be replaced by the default key serde as defined in config.

@param keySerde the key serde to use. If {@code null} the default key serde from config will be used
@param <K>      key type
@param <V>      value type
@param <VO>     other value type
@return new {@code Joined} instance configured with the keySerde
streams/src/main/java/org/apache/kafka/streams/kstream/Joined.java