API endpoint for fetching a single codelist

[mikerkelly]

Add an API endpoint to fetch a single codelist directly by slug.

There is no current client for this in the OpenSAFELY systems, but it seems plausibly useful to those interested in using the OpenSAFELY API in scripting and tooling. Part of the "Open" ethos is that the resources that are publicly human-accessible on the site should be conveniently accessible in machine-readable form via an API.

It would be useful in https://github.com/ebmdatalab/codeusage/, allowing it to avoid needing to decode the organization from the slug and fetch all codelists in the organization then find the specific codelist within the response. This is slowing down this tooling and slightly inefficient for our resource consumption.

[iaindillingham]

Thanks for this, @mikerkelly. From today's planning meeting, I believe you created it in response to speaking with Milan at the OpenSAFELY Symposium. It's not quite urgent or important enough to be addressed during the OpenCodelists initiative, but we can return to it during deck-scrubbing.

Before we do, we should consider whether the OpenCodelists API is public in the sense of "we expect it to be used by clients that weren't created by the tech team".

See also #2190