Add new DotNetBuildOffline switch at the orchestrator level

[mmitche]

See https://microsoft-my.sharepoint.com/:w:/p/mmitche/EdchH8A4okpOoKhkki1sJEYB0LyAaaCyX5_6A887gL744Q?e=JERvht for full design:

When “true”, remove non-local input sources. Infrastructure switch only. This switch is only exposed at the orchestrator level.
This replaces the existing DotNetBuildOffline switch.

Add a 'new' DotNetBuildOffline switch at the orchestrator level

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[mmitche]

There is already BuildWithOnlineSources. Using that would be good.

[mmitche]

dotnet/installer#18398