title: Uppercase characters not allowed in functions config variable names
body: <p>After executing this Firebase CLI command:
  <code>firebase functions:config:set osp.google.clientId=...</code></p>
  
  <p>I get: <code>Error: Invalid config name osp.google.clientId, cannot use upper case</code></p>
  
  <p>Given that <a href="https://firebase.google.com/docs/functions/config-env?authuser=0" rel="noreferrer">Firebase environment variables</a> are based on Google Runtime Configuration (GRC), why aren't them allowed?</p>
  
  <p>This <a href="https://cloud.google.com/deployment-manager/runtime-configurator/reference/rest/v1beta1/projects.configs.variables?authuser=0#Variable" rel="noreferrer">link</a> explains possible variable names in GRC, and it certainly allows uppercase characters.</p>
  
  <p>Not that it affects me, but it isn't properly documented in Firebase's docs.</p>