mrc-6113 JSON schema

[EmmaLRussell]

Add JSON Schema for JSON endpoint responses, and validate responses during integration tests.

The main controversy here is how to handle validating both our generic outer response format and the inner data property which changes for each endpoint, without repeating ourselves.

I was hoping to flashily get this working with $dynamicRef, to plug in different data schema into the outer "Response" schema - however I was scuppered by an outstanding bug in Ajv where dynamic refs result in spurious validation errors with "required"s being wrongly applied to the dynamic refs. Could potentially have got this working in hyperjump, but that didn't seem to reliably yield nice validation errors, so it didn't seem worth it...

So instead I'm using $ref - which requires a corresponding $id to match in an available data schema. This works as long as there is only one loaded schema with the relevant $id (which is grout-data). I could have put this $id in all the data schema, but it didn't quite sit right to have duplicate ids in the files themselves, so instead I'm appending the $id to the loaded data schema at runtime in the integration tests.

See the README update for more details on implementation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.41%. Comparing base (e31e8fd) to head (18e9203).
Report is 15 commits behind head on main.

Additional details and impacted files

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##             main       #6   +/-   ##
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  Coverage   98.41%   98.41%           
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  Files          15       15           
  Lines         126      126           
  Branches       15       15           
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  Hits          124      124           
  Misses          2        2           

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[M-Kusumgar]

Think it turned out pretty clean! shame about the bug though