Implementation of the KafkaConsumer (#24)

* First implementation of KafkaConsumer

Modifications:

* created a new .testTarget "IntegrationTests" with a dedicated
  directory
* made KafkaConsumerMessage and KafkaAcknowledgedMessage use
  KafkaPartition as type for the partition property
* implemented first consumer that enables users to subscribe to topics
  and poll the consumer
* built a KafkaSequenceConsumer on top of KafkaConsumer that enables
  users to consume messages in an AsyncSequence
* enforced that the integration-test-topic is created when in the Kafka
  docker container

Result:

Users are able to consume messages from a Kafka cluster

* Improved Public Interface for Consumer

Modifications:

* created Tests for KafkaConsumer
* updated Docker compose Kafka server to create different topics for
  different tests to avoid overlaps
* introduced two initializers for KafkaConsumer allowing to subscribe to
  a topic as a group or assign the KafkaConsumer to a particular
  topic+partition pair
* added ability to manually commit offsets to Kafka

* Implemented Backpressure into KafkaConsumer

Modifications:

* made KafkaConsumer an actor
* implemented backpressure for KafkaConsumer

* DocC Comments for KafkaConsumer

Modifications:

* made KafkaConsumer.close() method async instead of blocking

* Improve Concurrency Model for KafkaConsumer

Modifications:

* KafkaConsumer blocking operations are now executed on a serial
  `DispatchQueue`
* remove `deinit` as `close` is a blocking function and shall be invoked
  by the user anyway

* Changes Franz

Modifications:

* move from Swift 5.6 to Swift 5.7
* add offset property to assignment consumer initializer
* remove default config object in consumer initializer
* added preconditions in KafkaConsumer that check if functions are
  called on the serial queue
* make ConsumerMessagesAsyncSequenceDelegate a struct containing
  closures
* close KafkaConsumer on NIOAsyncSequenceProducerDelegate.didTerminate
  instead of a public close function
* add test utility functions to create and delete topics
* Docker: build librdkafka from source instead of installing from apt
  because apt only provides librdkafka version 1.2.1 whereas we use
1.9.2 currently
* repalce KafkaConsumer.State with simple boolean "closed" and
  assertions
* refactor KafkaConsumerMessage
* test IntegrationTests with multiple messages for each test

Author: felixschlegel

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.6
+// swift-tools-version: 5.7
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the swift-kafka-gsoc open source project
@@ -54,5 +54,9 @@ let package = Package(
             name: "SwiftKafkaTests",
             dependencies: ["SwiftKafka"]
         ),
+        .testTarget(
+            name: "IntegrationTests",
+            dependencies: ["SwiftKafka"]
+        ),
     ]
 )

Sources/SwiftKafka/KafkaAcknowledgedMessage.swift

@@ -24,15 +24,15 @@ public struct KafkaAcknowledgedMessage: Hashable {
     /// The topic that the message was sent to.
     public var topic: String
     /// The partition that the message was sent to.
-    public var partition: Int32
+    public var partition: KafkaPartition
     /// The key of the message.
     public var key: ByteBuffer?
     /// The body of the message.
     public var value: ByteBuffer
     /// The offset of the message in its partition.
     public var offset: Int64
 
-    /// Initialize `KafkaAckedMessage` from `rd_kafka_message_t` pointer.
+    /// Initialize ``KafkaAcknowledgedMessage`` from `rd_kafka_message_t` pointer.
     init(messagePointer: UnsafePointer<rd_kafka_message_t>, id: UInt) throws {
         self.id = id
 
@@ -57,7 +57,7 @@ public struct KafkaAcknowledgedMessage: Hashable {
         }
         self.topic = topic
 
-        self.partition = rdKafkaMessage.partition
+        self.partition = KafkaPartition(rawValue: rdKafkaMessage.partition)
 
         if let keyPointer = rdKafkaMessage.key {
             let keyBufferPointer = UnsafeRawBufferPointer(