Option to add key to KafkaSink

[kruscho]

Add an option to KafkaSink that allows users to specify a Kafka message key (static value, field-mapped value, or expression) so emitted records include a key. This enables consistent partitioning, message compaction compatibility, and easier downstream processing.

• Kafka partitions messages by key; many consumers and topics expect keys for ordering, compaction, or lookup.
• Current KafkaSink in StreamPipes emits messages without configurable keys, forcing users to implement workarounds (custom sinks, external producers).
• Providing a first-class key configuration keeps user flows simple and consistent with StreamPipes’ low-code goals.