Kafka support wishlist

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jet/equinox#87 provides an initial Kafka Producer/Consumer pair, and jet/dotnet-templates#11 provides a skeleton app using same.

There's infinite scope for improvement from this base. This Issue gathers an unordered wishlist and should not necessarily be taken as a roadmap.

Kafka Consumer:

• Lag monitoring needs to be integrated
• integrate FeedValidator usage into consumption side
• Checkpointing needs to interlink with Kafka offsets storage such that one can take a given consumption offset expressed in terms of a changefeed position + kafka offsets and pass that to a consumer working off a separate projector

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Moved here as

• producer/consumer logic lives in https://github.com/jet/Jet.ConfluentKafka.FSharp
• final surfacing in a sample lives in this repo

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Moved here as any such logic would likely be built here. Closing for now as main goal is accomplished by StreamConsumer, and lag monitoring is complete.