Allow serialization/deserialization of key

[nicolaferraro]

For Kafka usage, we should change the json serialize/deserialize to allow doing that on the key rather than the body.

If it does not complicate the interface, we may allow doing that on generic headers as well.

Wdyt @oscerd ?

[oscerd]

Yes, we can do that. By transforming the header json and re-insert it as transformed header, I guess.

[oscerd]

Lets park this for a while.

[oscerd]

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