Add support for serializing Avros' `GenericRecord` etc types

While maybe not the biggest problem in the world, there may be use cases where it'd be nice to let users essentially convert data bound in Avro-specific containers (like GenericContainer) into more sensible types (like basic Maps and Collections, say; or just POJOs).
One way to achieve this would be to add custom serializers for said types.

Another related feature would be the reverse (construct these types from streaming input, i.e add JsonSerializers); that can be a follow-up step if need be.