Implement the remaining `UserManagementService` methods

[da-tanabe]

The basic UserManagementService functions were implemented as part of #319; there are a few more to implement.

See https://github.com/digital-asset/daml/blob/9c03e7908bbd85ab1f8686cc581d49cc93beea92/ledger/ledger-api-client/src/main/scala/com/digitalasset/ledger/client/services/admin/UserManagementClient.scala for the full list.