PoC: Create a JSON schema SSOT for the `spec` keyword

Background

In #463328 (closed), we created a PoC for auto-generating a JSON schema for the spec keyword from the Ci::Header Entry classes. However, after some discussion, we decided it would be better to use a JSON schema as the source of truth and remove the Ci::Header classes entirely.

Work to be done

Create a PoC to demonstrate that it is feasible for us to use a JSON schema to validate the spec keyword (and all children) instead of the Ci::Header classes.