Add "updated_at" field to project-level environment variables
After running into https://circleci.com/blog/january-4-2023-security-alert/, contexts API now exposes "created_at" and "updated_at" fields on respective environment variables in contests (cf. https://circleci.com/docs/api/v2/index.html#operation/listEnvironmentVariablesFromContext)
While it is significantly useful to decide on which values need to be rotated, it'd be nice if CircleCI users can know equivalent information on project-level environment variables at https://circleci.com/docs/api/v2/index.html#operation/listEnvVars
Both of these are a big ++ for consistency across the API endpoints & it would help us manage issues like the recent incident in the future.