Support project level var sets for migration

[jeffmccollum]

It is now possible to create Var sets at the project level instead of org level. tfm should be able to copy a varset during a migration and put it at the project scope instead of org scope.

[alexbasista]

+1 on this.

I think the default functionality should be: copy the variable set keeping the same data.relationships.parent.data.type (either "organizations" or "projects").

It would also be great to have a flag for --destination-parent-type as there will be many use cases to convert an organization-owned varset into a project-owned (in the spirit of organization consolidation), and the destination Project ID should already be present in the TFM config for these cases.