Secrets migration for GH->GH migrations

[dylan-smith]

We did a PoC of how to migrate secrets here: https://github.com/dylan-smith/gh-secrets-migrator

We could include this functionality in the GEI CLI as a post-migration step when migrating GH->GH. It would be an additional option when running generate-script that would call a new migrate-secrets command.

[toddoconnor]

@dylan-smith I think this would address one of the points I made in #458.

In case we don't know if there are secrets or not, could a log message be printed something to the effect:

[12:00 AM] [WARN] Secrets were detected but not migrated. Please run the post-migration step XYZ to migrate your secrets.