Add Offset Begin/End/Stored/Invalid constants
Add Offset Begin/End/Stored/Invalid constants

MaximGurschi/rdkafka-dotnet,treziac/confluent-kafka-dotnet,MrGlenThomas/confluent-kafka-dotnet,treziac/confluent-kafka-dotnet,bjornicus/confluent-kafka-dotnet,bjornicus/confluent-kafka-dotnet,ah-/rdkafka-dotnet,MrGlenThomas/confluent-kafka-dotnet