Direct Transfer - Contributions of ghost user should be directly assigned to ghost user on destination

Similar to Contributions of ghost user should be directly ... (#508110 - closed), imported contributions assigned to the ghost user on the source should be mapped straight to the Ghost User in the destination.

Proposal solution

1. Update Direct Transfer to request the Ghost User ID from the source instance. For this we will have to make a GraphQL request

     {
       user(username: "ghost") {
         id
       }
     }

2. Update Direct Transfer not to create placeholder users for contributions associated with the Ghost source user ID

3. Update Direct Transfer to assign the contributions to the Ghost destination user ID and not push placeholder user references

Note that we will need a different approach to figure out the Ghost User ID for air-gapped migrations since we can't make HTTP requests.