User merge & migration support on LoginCluster federations
/arvados/v1/user/merge operation isn't supported on federated scenarios.
One of the main issues that need handling is that the account merging includes the migration of objects owned by the old source to the target account. The migration is easily doable when the user requests the merging on a 2-cluster federation (a login cluster and a satellite cluster), but notifying the rest could be cumbersome if there's more than one satellite cluster.
One option could be to flag the user account somehow as recently merged so that when the user logs into the satellites, an on-demand local migration is automatically performed.