Key-Value Schema

[gabestein]

Motivation

So devs can add a key-value schema to a relationship field via the API
In the near future, so admins can specify a schema for the value of a relationship field, e.g. that a contributor needs a role

Requirements

• Ability to select a “key-value” schema for a field when creating a field
• Ability to add this schema to a relationship field value via the API

Acceptance Criteria