Make storage engines pluggable

[willdady]

Currently contentjet-api comes with a single storage engine which writes files to disk. This is fine for simple setups but limits the app from scaling horizontally. Need to provide a way to load and configure storage engines for writing to different storage providers such as AWS S3 etc.

Some thoughts which are by no means final:

• Storage engines should be installable via NPM and adhere to a defined API.
• Considering the app is distributed via Docker images users would have to npm install inside their container or extend the base image. Is that too hard?