Add lookup for CEFACT code

[ferrisoxide]

Per DEV NOTE from #113 (see below), add an endpoint for looking up CEFACT Annex II codes used with unitCode attributes.

See

https://github.com/uncefact/vocab-codes/blob/main/CLR/Rec20/current/Annex%20%7C%7C%20%26%20Annex%20%7C%7C%7C/code-list.csv

---

DEV NOTE

Per Using-UN-CEFACT-Codes, for units of measurement we will use UN/CEFACT-Rec20, see:

https://digitallibrary.un.org/record/635432?v=pdf
https://unece.org/fileadmin/DAM/cefact/recommendations/rec20/rec20_rev3_Annex3e.pdf

rec20_Rev17e-2021_0.xlsx

We may be able to import directly from another repository for the CEFACT data, see:

https://github.com/uncefact/vocab-codes/blob/main/CLR/Rec20/README.md

or

https://github.com/CIFConsulting/unece-recommendation-20

Originally posted by @ferrisoxide in #113