Get a list of all coworkers' calendars
<p><em>There seems to be a <a href="https://stackoverflow.com/questions/19731102/get-all-google-calendars-shared-within-a-domain">similar question</a> already, but I need a solution (read on) that authenticates as the current user.</em></p>

<p>I'm trying to make a simple Google Apps Script app that uses Google Calendar to check if your coworker (within a Google Apps for Work domain) is currently in a meeting.</p>

<p>The script would execute as the current user. To do this, I need to be able to get a list of <strong>all calendars available to the current user</strong> (not just ones they've subscribed), the same list that's available in the dropdown in the standard Calendar web app:</p>

<p><a href="https://i.stack.imgur.com/ZSeEd.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ZSeEd.png" alt="google calendar look up coworkers&#39; calendars"></a></p>

<p>The <a href="https://developers.google.com/apps-script/reference/calendar/calendar-app#getallcalendars" rel="nofollow noreferrer">CalendarApp.getAllCalendars()</a> gets all calendars <em>that the user owns or is subscribed to</em>, which is not what I need. I want the <em>full list of calendars</em> <strong>available</strong> to me within my Google Apps for Work domain (e.g. shared with the entire company), even if I'm not subscribed to them. </p>

<p>The <a href="https://developers.google.com/google-apps/calendar/v3/reference/calendarList/list" rel="nofollow noreferrer">CalendarList REST API</a> available to Apps Scripts via <a href="https://developers.google.com/apps-script/advanced/calendar" rel="nofollow noreferrer">Advanced Calendar Service</a> has the same limitation.</p>

<p>The page <a href="https://developers.google.com/google-apps/calendar/concepts/domain" rel="nofollow noreferrer">Calendar for Work Features</a> also mentions <em>domain-wide authority delegation</em>:</p>

<blockquote>
  <p>Accessing domain calendars as an app</p>
  
  <p>An app can access domain-owned calendars without requiring user
  credentials if it authenticates using a service account. The service
  account must have the necessary access using domain-wide authority
  delegation.</p>
</blockquote>

<p>which may get me what I need, but I don't want my script to run with admin rights, I want it to authenticate as the current user accessing it.</p>

<p>Is there a way?</p>