Support adding default values to generated schema

@dstelljes I’m creating this issue based on your comment on #7

Since default values aren’t accessible via reflection, maybe an easier way to implement this functionality would be via an annotation? Something to the effect of

public class Message
{
  [AvroDefaultValue(null)]
  public int? Property { get; set; } = null;
}

[polanski-piotr]

@dstelljes Hey, I have suggestion.

Since our event models contains domain models, the Kafka related attributes on domain model will create tight coupling to our infrastructure (Avro schema builder in this case) and this usually not a good thing.

Maybe instead of attributes on model we should add some configuration options to schema builder, something like

eventModelSchemaBuilder.Create<SomeEventModel>()
  .MakeOptional(em => em.Something.Inside.Model)
  .MakeOptional(em => em.SomethingElse.Inside.Model)
.Build()

[dstelljes]

Support for the [DefaultValue] attribute is released with 8.0.0-pre.1.