Add support of multi producer configuration

[ShapovalovaIrina]

Hi,

Thanks for the great library.

In my projects, I encountered the need to send messages to different Kafka instances. Also there was an issue #155 for this functionality.

Configuration

Similar to multiple consumers #131, I added the ability to configure multiple producers:

config :kaffe,
  producers: %{
    "producer_1" => [
      endpoints: [kafka1: 9092], # [hostname: port]
      topics: ["kafka-topic"],
  
      # optional
      partition_strategy: :md5,
      ssl: true,
      sasl: %{
        mechanism: :plain,
        login: System.get_env("KAFFE_PRODUCER_USER"),
        password: System.get_env("KAFFE_PRODUCER_PASSWORD")
      }
    ],
    "producer_2" => [
      endpoints: [kafka2: 9092], # [hostname: port]
      topics: ["another-kafka-topic"]
    ]
  }

Single producer configuration with :kaffe, :producer is preserved.

Message producing

To ensure backward compatibility with a single client, the produce_sync_with_client function was added, where the first argument is the producer name. The remaining arguments are identical to the produce_sync function.

Examples:

Kaffe.Producer.produce_sync_with_client("producer_1", "topic", [{"key1", "value1"}, {"key2", "value2"}])

Kaffe.Producer.produce_sync_with_client("producer_2", "key", "value")