BP-45: a pluggable way to modify payload sent to the ledger

**BP**

> Follow the instructions at http://bookkeeper.apache.org/community/bookkeeper_proposals/ to create a proposal.

This is the master ticket for tracking BP-45 :

The proposed change targets addition of a pluggable way to modify payload sent to the ledger. Specific use cases include GDPR compliance, encryption, and compression. While these can be implemented at the application level, having this as a BK client level extension unlocks such functionality to any application built on top of Bookkeeper.

Proposal PR - https://github.com/apache/bookkeeper/pull/2730