Add backfill functionality to official extensions

[huangjeff5]

[READ] Step 1: Are you in the right place?

Issues filed here should be about a feature request for a specific extension in this repository. To file a feature request that affects multiple extensions or the Firebase Extensions platform, please reach out to Firebase support directly.

[REQUIRED] Step 2: Extension name

This feature request is for extension: _ (firestore-translate-text, firestore-shorten-urls-bitly, auth-mailchimp-sync)

What feature would you like to see?

These extensions need backfill functionality added. We will have a best practice for a few of the other official extensions to follow as a guide (will update this bug as soon as that's ready), but these need to be enhanced to add that functionality.

How would you use it?

Tell us how you'd use this feature in your app.