Minor Description
In working with Kafka Streams I've found it's often the case I want to perform a "group by" operation, where I repartition a stream based on a foreign key and then do an aggregation of all the values into a single collection, so the stream becomes one where each entry has a value that is a serialized list of values that belonged to the key. (This seems unrelated to the 'group by' operation talked about in KAFKA-3544.) Basically the same typical group by operation found in systems like Cascading.
In order to create these intermediate list values I needed to define custom avro schemas that simply wrap the elements of interest into a list. It seems desirable that there be some basic facility for constructing simple Serdes of Lists/Maps/Sets of other types, potentially using avro's serialization under the hood. If this existed in the core library it would also enable the addition of higher level operations on streams that can use these Serdes to perform simple operations like the "group by" example I mention.