Make authentication configurable

Problem/Motivation

This code was initially written for a client-facing project, which meant it was possible to take shortcuts like this to get everything running:

  private function getAirtableApiKey() {
    return $_ENV['AIRTABLE_API_KEY'];
  }

For contrib, we should follow a best practice for managing credentials, Key?? → - and replace the contents of getAirtableApiKey with something that returns the secrets-managed credentials.