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

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

```console
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

Start by reading the image_config() implementation in client.rs and compare its ImageConfiguration parsing with the OCI Artifact configuration described in the linked Wasm OCI Artifact specification. Trace how the stress test selects and parses the target image, then make completion mean that the cited OCI artifact no longer fails with a missing rootfs error while standard container images continue to work.

Written by the indexing model from the issue text.