ISSUE-2731: BP-45: a pluggable way to modify payload sent to the ledger

[sijie]

Original Issue: apache#2731

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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 - apache#2730