Allow `@AutoPopulated` fields to preserve supplied values

[kevin-wise]

Feature description

There are situations where you might want to provide a value for a field which is ordinarily auto-populated. For example if you want to backfill historical data from another source, you might want to use the create time as of the time when it was created in the other system. Another example might be backfilling records with a UUID primary key from a legacy system.

Right now to allow for that, you have to remove the AutoPopulated or DateCreated annotations. It would be really nice to add an option to the annotations to keep any values which are supplied. Something like @DateCreated(overwriteSuppliedValues = false) or maybe @DateCreated(keepNonNullValues = true).