Refactor KafkaProducer

Motiviation:

* align `KafkaProducer` more with proposed changes to `KafkaConsumer`
* `AsyncStream` was not handling `AsyncSequence` termination handling as
  we wanted it to, so revert back to use `NIOAsyncSequenceProducer`

Modifications:

* make `KafkaProducer` `final class` instead of `actor`
* `KafkaProducer`: use `NIOAsyncSequenceProducer` instead of
  `AsyncSequence` for better termination handling -> shutdown
  `KafkaProducer` on termination of the `AsyncSequence`
* introduce `StateMachine` to `KafkaProducer`
* move internal state of `KafkaProducer` to `KafkaProducer.StateMachine`
* remove unused `await` expressions when accessing `KafkaProducer`
* update tests
* update `README`

README.md

@@ -11,7 +11,7 @@ The `sendAsync(_:)` method of `KafkaProducer` returns a message-id that can late
 ```swift
 let config = KafkaProducerConfiguration(bootstrapServers: ["localhost:9092"])
 
-let (producer, acknowledgements) = try await KafkaProducer.makeProducerWithAcknowledgements(
+let (producer, acknowledgements) = try KafkaProducer.makeProducerWithAcknowledgements(
     config: config,
     logger: .kafkaTest // Your logger here
 )
@@ -25,7 +25,7 @@ await withThrowingTaskGroup(of: Void.self) { group in
 
     // Task receiving acknowledgements
     group.addTask {
-        let messageID = try await producer.sendAsync(
+        let messageID = try producer.sendAsync(
             KafkaProducerMessage(
                 topic: "topic-name",
                 value: "Hello, World!"