Add support for Github Enterprise for source repositories (CE Migration)

[nphilbrook]

For a Community Edition migration, currently Github support for cloning source repositories is limited to Github.com - it would be great to add support for self-hosted Github Enterprise.

Many ways to implement this, but here's one

https://pkg.go.dev/github.com/google/go-github/v67/github#Client.WithEnterpriseURLs

[nphilbrook]

As community edition migration effort has been moved to tf-migrate we should consider closing this.