Add SMT to extract inner value field

Accessing inner values of records (inside before/after, depending of the event type) in other transforms (e.g. to change the records key) is currently not possible without applying event flattening with io.debezium.transforms.ExtractNewRecordState.

It would therefore be great if Debezium comes with a transform that allows extracting a field from the inner value (from before on delete operation, otherwise from after without the need to loose information through flattening.

A PR that adds such a transform is here: https://github.com/debezium/debezium/pull/967