Activate this mode by passing an FCreateArchive to InitializeAsync. PreloadHandle: A synchronous inner archive is opened using the passed-in CreateArchive function; this call is made asynchronously on a TaskGraph thread. Optionally, a precache request is sent to the inner archive for the first bytes; this call is also made asynchronously. The created and primed lower-level FArchive can then be detached from this class and handed off to a new owner.