Transfer: Push: fix failure on pushing duplicated blobs

[ktock]

Currently transfer service doesn't handle conflict among pushing of duplicated blobs and pushing an image with duplicated blobs fails.

This commit fixes this issue by using dockerPusher.Writer that detects the conflict and allow retrying using content.OpenWriter helper. content.OpenWriter is already used by remotes.PushHandler to handle duplicated blobs safely.
But transfer service doesn't use this so it (and ctr i push) fails to push duplicated blobs.

This commit fixes this issue.

Reproduce

Pushing of gcr.io/k8s-staging-cri-tools/test-image-tags:1 (contains duplicated manifests, as of now) to another registry fails without this patch. (Detected at stargz-snapshotter CI: https://github.com/containerd/stargz-snapshotter/actions/runs/9303215973/job/25605036366?pr=1545#step:4:10911 )

ctr i pull --all-platforms gcr.io/k8s-staging-cri-tools/test-image-tags:1
ctr i tag gcr.io/k8s-staging-cri-tools/test-image-tags:1 registry2:5000/test-image-tags:1
ctr i push --plain-http registry2:5000/test-image-tags:1