stress-test: OCI Artifact support fails due to missiing rootfs in image configuration

[Mossaka]

The stress test tool currently fails when trying to test with OCI artifacts such as ghcr.io/conta inerd/runwasi/wasi-demo-oci-artifact:latest due to an assumption about the image configuration format.

It failed with

Error: serde failed

Caused by:
    missing field `rootfs` at line 10 column 1

The image_config() implementation in client.rs attempts to parse all the images using ImageConfiguration which expects a standard container image with a rootfs field.

However, OCI Artifacts have a different configuration format. See https://tag-runtime.cncf.io/wgs/wasm/deliverables/wasm-oci-artifact/

Suggested solutions

1. Detect if the target is an OCI artifact and use the appropriate parsing