Expose masked value of environment variables in contexts
CircleCI's web UI is exposing last 4 digits of the value in environment variables of contexts, however, the equivalent information isn't available via https://circleci.com/docs/api/v2/index.html#operation/listEnvironmentVariablesFromContext
Unlike contexts's environment variables, API for project-level environment variables are displaying last 4 digits of those sensitive values for help inspecting (cf. https://circleci.com/docs/api/v2/index.html#operation/listEnvVars). It's nice if contexts API also behaves similar way.