Add a dead letter queue (DQL) implementation

Implemented as a draft extention of termainl retry handling:

https://github.com/astubbs/parallel-consumer/pull/8

Extension of:
- #242

Related:
- https://github.com/confluentinc/parallel-consumer/issues/196

# Workaround
Implement it yourself by producing to a dlq topic when you think you’ve retried too many times, and then don’t throw an exception so PC thinks it was successful

Review the draft extension in pull request #8 and the linked work in #242, then check the closed pull request #366 and related issue #196 for the current state. No files or tests are named; done would mean a settled dead letter queue implementation extending terminal retry handling rather than relying on the documented workaround.

Written by the indexing model from the issue text.