feat: add support for managedPrivateNetwork in AwsS3Data storage transfer jobs

[scottclay]

storagetransfer: added `private_network` field to `google_storage_transfer_job` resource

Adds support for managedPrivateNetwork field from the API that allows transfers from AWS to GCP using the google private network.

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[scottclay]

Duplicate of #13379 it seems

It's not clear to me how we should test this.

I can't see any of the existing AWS stuff tested in here but maybe I'm missing something?

I've tested it by building the provider locally and spinning up a STS job between AWS and GCP accounts/projects we own.