withStorage - allow arbitrary and composite PKs

[adrians5j]

Introduction

Currently, the withStorage layer works with the id field as the entity's PK. The value of the id field is always a simple MongoDB ID.

This is not good for a couple of reasons.

1. In some databases, like e.g. DynamoDB, the user basically never uses a randomly generated ID for PKs. So, this means we need to ensure that users can use any arbitrary string as the PK.

2. Users cannot define a composite PK - a key that consists of two or more fields.

The solution

We need to allow users to use any arbitrary string as the PK, and we need to allow defining composite PKs.

Implementation details

TBD