Support key (de-)serializers

[gunnarmorling]

See #1 (comment)

[nicoweidner]

Some more flavor on this issue: Key (de-)serializers are required to support writing to Change streams and Append streams with partition key.

As part of this issue, make sure the SDK determines based on stream properties whether the target stream is a change stream or append stream, and validates that a key (de-)serializer is set if it's a change stream. Throw an exception at runtime if no (de-)serializer is set.