Add blob count to image pull progress display

[Copilot]

Users pulling images with ctr had no visibility into how many components (manifests, configs, layers) were being processed.

Changes

• Added countBlobs() to recursively count all progress nodes in the transfer tree
• Modified DisplayHierarchy() to display the blob count in the status line

Output

Before:

Completed pull from OCI Registry (docker.io/library/alpine:latest)	elapsed: 1.3 s	total:  29.9 K	(22.9 KiB/s)

After:

Completed pull from OCI Registry (docker.io/library/alpine:latest)	elapsed: 1.3 s	total:  29.9 K	(22.9 KiB/s)	blobs: 35

The term "blobs" encompasses all registry objects being transferred: manifests, configs, and layers.

Original prompt

I want you to work on solving this issue containerd#10046. Can you please help out?

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