Resourceadm: show number of delegations in Altinn 2 for imported link services

[mgunnerud]

If a link service is imported from Altinn 2, the Migration tab will be visible. It should be possible to see the number of delegations in Altinn 2 for a service imported from Altinn 2 here.

### Tasks
- [ ] Add new controller endpoint for loading number of Altinn 2 delegations in bff
- [ ] Call new controller endpoint in Migration tab to load number of Altinn 2 delegations for a service