We recently migrated from Google Analytics 4 to Amplitude, and all of our users are now being considered as new, even if they're existing users. We'd like to maintain the continuity of our user data. Is it possible to import historical data or backfill key interactions to preserve user history? If so, what's the best approach to do this, especially for the most important user actions? Are there any limitations or best practices we should be aware of when attempting this kind of data import?