Support for sha mapping file to update PRs during migration

## Description

Before migration from GHES to GHEC, a customer with large files in a repo must first migrate those large files to LFS. This rewrites the git repo history, affecting PRs. The PR shas will be outdated. You can output a csv mapping file of the old shas to the new ones using `git lfs --object-map=` command. And if using ECI, a user can manipulate the pull_request json files in the tarball to rewrite the previous shas with the new ones.

Many migrations into GitHub cloud require LFS migration. GEI should support a smooth way for customers to provide this mapping file generated from `git lfs migrate` in order to update the PRs during migration.