title: Is there no continuation action "ContinueWithOnMainThread" in 6.12.0 unity sdk?
body: <p>I'm using firestore latest sdk "6.12.0" on unity. I've to get the data from the database and save it to PlayerPrefs but because PlayerPrefs only run on the main thread, it is causing an error to go with older "ContinueWith" continuation action.</p>
  
  <p>Firebase has given this "ContinueWithOnMainThread" in the doc but I'm unable to find it on API. I only get the option of "ContinueWith".</p>
  
  <p>What's the issue, does "ContinueWithOnMainThread" exist in 6.12.0?</p>