feat(shim): add task wrapper and containerd access session

[sidneychang]

Description

Introduce a urunc-specific containerd shim task service wrapper and a small
shim-side containerd access foundation.

The task plugin now wraps containerd/runc's task service through a urunc-owned
task service type. For now, the wrapper is behavior-preserving and forwards
Create/Delete to the wrapped runc service. This gives later shim-side features a
single shared hook point instead of adding independent task plugins.

The containerd access layer adds a narrow containerdSession helper that owns a
short-lived gRPC connection, namespace propagation, task-level container metadata
lookup, gRPC error conversion, and generated service client construction for the
containerd APIs needed by the annotation and snapshot-view work.

This intentionally does not wire the annotation-injection or snapshot-view
logic yet. Those feature helpers can build on this foundation while keeping
feature-specific behavior outside the common containerd access layer.

Related issues

• Refs Come up with a common interface for the sim communication with contianerd #642
• Related to Feat/inject missing annot in shim #565
• Related to feat(view): add per-container snapshot views for boot artifacts #639
• Related discussion: LFx Mentorship (urunc Optimize rootfs handling with block-based snapshotters) #523

How was this tested?

LLM usage

GPT-5

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint).
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the llm policy.

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[sidneychang]

Hi @cmainas and @jiwahn,

I opened this draft PR for the initial task service wrapper and narrow containerd access session.

There are still some implementation details to clean up, but I would like to first check whether the overall design direction looks reasonable to you.

Please let me know if this layering matches what we discussed, or if you think I should adjust the structure before continuing with the remaining details.

Thanks!