Add a standard message type.

[ddebrunner]

The operators default to looking for a schema with key and message, it would be good to have such a type ready to use in this and the kafka toolkit.

[cancilla]

What would the attribute types be for key and message since the operators support multiple types for keys and messages.

[ddebrunner]

How many combinations are there, four ? (rstring,blob) x (key,message) ?

[cancilla]

Both the key and message support any combination of the following types:

• rstring,
• int32/int64
• uint32/uint64
• float64
• blob

[ghost]

What would be the most used combinations? I guess these both:
type StringMessage = tuple <rstring message, rstring key>;
and
type BlobMessage = tuple <blob message, rstring key>;

In my opinion, all other SPL types are rarely used in Kafka messages for key or value.

[ghost]

MessageType.StringMessage = rstring message, rstring key;
MessageType.BlobMessage = blob message, rstring key;
MessageType.ConsumerMessageMetadata = rstring topic, int32 partition, int64 offset, int64 messageTimestamp;
MessageType.TopicPartition = rstring topic, int32 partition;

implemented in 1.7.0 release