Support Binary Payloads in V2

[rido-min]

MQTT does not use, or imply, any text encoder to pub/sub messages. Although the most common messaging serialization format is JSON string encoded as UTF8, will be good to allow custom BinarySerializers such as AVRO, Protobuf, or CBOR.

Here is a proposal for a customizable PayloadBinarySerializer

public abstract class PayloadBinarySerializer
{
    public abstract byte[] ToBytes<T>(T payload);
    public abstract T FromBytes<T>(byte[] bytes);
}

By default, the SDK can provide a JSONUtf8PayloadConvention like:

public class JsonUtfSerializer : PayloadBinarySerializer
{
    public override T FromBytes<T>(byte[] bytes) => JsonSerializer.Deserialize<T>(Encoding.UTF8.GetString(bytes))!;
    public override byte[] ToBytes<T>(T payload) => Encoding.UTF8.GetBytes(JsonSerializer.Serialize(payload));
}

When using other Binary formats, is very common to require a type definition, aka schema (usually produced by codegen tools) that must be provided before converting to/from bytes.

public class ProtobufSerializer : PayloadBinarySerializer
{
    private readonly Google.Protobuf.MessageParser? _parser;
    public ProtobufSerializer(Google.Protobuf.MessageParser parser) => _parser = parser;
    public override T FromBytes<T>(byte[] bytes) => (T)_parser!.ParseFrom(bytes);
    public override byte[] ToBytes<T>(T payload) => (payload as IMessage).ToByteArray();
}

[rido-min]

this sample shows Json, Proto and Avro serializers https://github.com/ridomin/Payloads