Make IAvroDeserializerImpl, IAvroSerializerImpl interfaces public to implement generic avro serdes

[keithazzopardi]

Description

I am sending multiple events on the same topic. Current confluent dotnet package is strongly type and initializes everything in the constructor. I have managed to implement a dynamic one for ISpecificRecord (We are only using ISepcificRecord) and had to implement a more generic IAvroDeserializerImpl, IAvroSerializerImpl. The problem however is that these interfaces are internal. Could you please make these public?

Checklist

Please provide the following information:

• [x] A complete (i.e. we can run it), minimal program demonstrating the problem. No need to supply a project file.
• [ ] Confluent.Kafka nuget version.
• [ ] Apache Kafka version.
• [ ] Client configuration.
• [ ] Operating system.
• [ ] Provide logs (with "debug" : "..." as necessary in configuration).
• [ ] Provide broker log excerpts.
• [ ] Critical issue.

[mhowlett]

we really need a good out-of-the-box solution to the problem you're trying to solve... for now, you'll need to resort to copy/paste - I don't think we want to make these interfaces public.