feat: AVRO with Schema Registry

[brunodomenici]

Hello,

I have a requirement to do AVRO with Schema Registry so, I'm proposing this implementation.

Few things:

• I had to upgrade confluent-kafka-go/v2
• I had to create a struct for Metric object. Confluent's serializer can't serialize map[string]interface{} (https://github.com/confluentinc/confluent-kafka-go/issues/1367)
• This branch is based on #135

Thank you