Feature: Implement producer with middleware support

[cyberhck]

with the middleware, we'll be able to plug various things, we can get away with just a raw producer without any serialization for now.

Once we have a raw producer, we can then introduce a producer that respects schema out of the box.

Acceptance Criteria:

• Can initialize a producer
• Can add middleware to that producer
• The signature of the middleware should be same as core
• Can produce *kafka.Message
• Can Flush(time.Duration)
• Can Close()

Here's how I want to initialize a producer:

package main

func main() {
    messageProducer := producer.New(getConfluentProducer())
    messageProducer.AddMiddleware(tracing.Middleware()) // don't worry about actually creating this middleware
    messageProducer.AddMiddleware(block_storage.Middleware() // don't worry about actually creating this middleware
    messageProducer.Produce(&kafka.Message{})
    messageProducer.Flush(time.Milliseconds * 100)
    err := messageProducer.Close()
    if err != nil {
        panic(err)
    }
}