Map Mannequins during migration

## Description

As an enterprise customer, we have over 7000 engineers actively using GHES. Emailing all 7000 to re-attribute is toilsome, especially when we already have GH Connect enabled and all of the on-prem identities are already linked to GHEC identities.

Request to implement previous features from ECI and legacy import tooling that allowed mapping/matching of mannequins either through a `user.csv` or other mapping file DURING import, rather than having to re-attribute them all later to claim mannequins.