As a maintainer of Castellan, I want to use a well-maintained library to implement the Hashicorp Vault backend so that I don't have to maintain both Castellan and a custom Vault client in the Castellan repo.

For this story we want to create a new backend that re-implements the Castellan interface using HVAC (Hashicorp Vault API Client).  The goal of this new implementation is to be a transparent drop-in replacement for the existing bespoke vault implementation.  It's imperative that this implementation behave exactly like the existing one so that data that was stored using the existing implementation can be retrieved with the new implementation.