SERVER-98581 introduced a helper for sending aggregate command to donor shards to get the number of documents to copy. The helper sends the aggregate command via the ShardRemote, which is an anti-pattern since the rest of the ReshardingCoordinatorService sends commands to donors and recipients via the ReshardingCoordinatorExternalState::sendCommandsToShards (i.e. sharding_ddl_util::sendAuthenticatedCommandToShards) which uses async_rpc::sendCommand and sends the commands in parallel.

We should make the helper use ReshardingCoordinatorExternalState::sendCommandsToShards, which does support retries. It currently doesn't custom readPreference (it is always set to primaryOnly) and doesn't support attaching shardVersion but that can be changed. 

As part of this, we should refactor the helper to allow SERVER-100358 to re-use some of the code in there.