Couchbase Lite: Encryption API Replicator Hook for client side encryption

*mini PRD: https://docs.google.com/document/d/1TTmI-hPmOnx4FLNWUQN825lCFT8tX61qqx16gcNi_BM/edit?usp=sharing*

PRD: WIP

Couchbase Lite is increasingly used in applications dealing with sensitive data where there are stringent requirements w.r.t data privacy and compliance with data governance policies. This is especially true in healthcare and finance sectors. In such applications, it is imperative that PII data is encrypted and inaccessible to actors other than the clients that are consuming or generating the data.

The ability for CBL apps to encrypt sensitive fields in document with suitable encryption library of choice would be required.

This is ticket for tracking interest