New message transformation concept

[TiMESPLiNTER]

This PR aims to deprecate the current Serializer approach and replaces it with a "message transformation concept" which allows to modify the message before after consumption before sending. (For example to de/encode the payload if it's in Avro format.)

This PR should not introduce any BC.

I think the Serializer as of today is really just a workaround for transporting the message headers back when kafka didn't had any support for native headers. As we have it now I think we either should deprecate it and remove it as soon as possible (next major) or replace it with a proper concept (which this PR is a proposal for along with deprecating the Serializer).

wdyt?

[Steveb-p]

@TiMESPLiNTER Actually you might want to look into #846 and PRs/issues linked in it's description.

Long story short there was a discussion about something similar to what you're proposing, but with the intention of allowing passing arbitrary types as messages to allow custom made serialization on any desired level.

I'll look into your PR further when I've got spare time if you dont mind :smile:

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