code: public CacheConfigurationBuilder<K, V> withKeySerializer(Serializer<K> keySerializer) {
      return withSerializer(new DefaultSerializerConfiguration<>(requireNonNull(keySerializer, "Null key serializer"), DefaultSerializerConfiguration.Type.KEY));
    }
docs: Adds a {@link Serializer} for cache keys to the configured builder.
  <p>
  {@link Serializer}s are what enables cache storage beyond the heap tier.
  
  @param keySerializer the key serializer to use
  @return a new builder with the added key serializer