Add `--ado-server-url` to `ado2gh migrate-repo` and `ado2gh generate-script`

[dylan-smith]

We currently have support for --ado-server-url in gh gei for on-prem ADO Server migrations. However, we do not have this option in gh ado2gh.

We'll need to add this option to ado2gh before we can entirely remove the ADO code paths from gh gei (currently the ADO options are hidden in gh gei but not removed).

It should be added as an option to both migrate-repo and generate-script. The option should be hidden (as it is in gh gei) since we do not officially support migration from ADO Server.

Any customers that rely on this functionality in gh gei should continue using gh gei and ignore the warning to switch to gh ado2gh for now. They should also subscribe to this issue, and once completed they should switch over to gh ado2gh.