Add examples for transactional producer and consumer

[dcrodman]

Hi @twmb! I'm implementing a new producer with this library since it seems to have great support for producing messages in transactions and I need exactly-once semantics. I ended up crafting a couple of examples to test things out and figured I might put them out there for the sake of consolidating some of the transaction documentation.

I know the other examples don't use a main package but I found it convenient to be able to run the example as-is so I left it in there. More than happy to make changes to that or any other part of this.

As a side note, if I'm doing anything wrong here I'd really appreciate any feedback or guidance here.

Example output (repeats every 10s):

produced message: batch 0 record 0
produced message: batch 0 record 1
produced message: batch 0 record 2
produced message: batch 0 record 3
produced message: batch 0 record 4
produced message: batch 0 record 5
produced message: batch 0 record 6
produced message: batch 0 record 7
produced message: batch 0 record 8
produced message: batch 0 record 9
consumed record with message: batch 0 record 0
consumed record with message: batch 0 record 1
consumed record with message: batch 0 record 2
consumed record with message: batch 0 record 3
consumed record with message: batch 0 record 4
consumed record with message: batch 0 record 5
consumed record with message: batch 0 record 6
consumed record with message: batch 0 record 7
consumed record with message: batch 0 record 8
consumed record with message: batch 0 record 9

[twmb]

Hey there, thanks for the example! I've added a few small comments to clarify some underlying behaviors, but otherwise this generally looks good!

[twmb]

Looks great, thank you!

[dcrodman]

Thanks for the suggestions and feedback!