Use a github variable containing 1password secret references

[jcpitre]

Describe the problem

In MobilityData/gtfs-validator#1921 we introduced a Github variable that contains a list of key value pairs for 1password secret references.
The goal was to be able to change (e.g. move vault) the entry referred by the secret reference without having to change the code.
We should do the same for mobility-feed-api

Ideally the action used to extract the secrets (extract-1password-secret/action.yml) will be made in a standalone reusable action.