Use comma delim server list in kafka sink

This change achieve consistency with our kafka source and other Kafka clients

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The kafka sink field bootstrap_servers has been changed from an array to a string, expecting a comma separated list of bootstrap servers similar to the kafka source.

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 [sinks.my_sink_id]
   type = "kafka"
   inputs = ["my-source-id"]
-  bootstrap_servers = ["10.14.22.123:9092", "10.14.23.332:9092"]
+  bootstrap_servers = "10.14.22.123:9092,10.14.23.332:9092"