oci: config entrypoint

Explore configuring an entrypoint in the OCI config file for a Spin app.

Originally from https://github.com/fermyon/spin/pull/1882#discussion_r1377004250:

Spin currently ignores this value, runwasi parses it and sends to the shim via RuntimeContext but it is ignored by spin's shim: https://github.com/deislabs/containerd-wasm-shims/blob/6ca8df8247d94cf1dc6ce5e529b00ee728aee4ec/containerd-shim-spin-v1/src/engine.rs#L123

The reason this is working when packages as a container is because / is the command is set on the yaml: https://github.com/deislabs/containerd-wasm-shims/blob/6ca8df8247d94cf1dc6ce5e529b00ee728aee4ec/README.md?plain=1#L95

In the case of a shim such as wasmtime/wasmedge it makes sense to have pass the entrypoint info as arg0 and we even do some additional parsing to allow for entering other functions in the wasm module https://github.com/containerd/runwasi/blob/642cafacde77d25762c8c0c0ca78ff1010caff16/crates/containerd-shim-wasm/src/container/context.rs#L17-L25

So in this case we can continue to use / in the command for the pod yaml or you could set this value in the config (maybe image name?) and users wouldn't need to set the command to / in the yaml. In the future maybe it can be useful otherwise we can revisit this in the next iteration of the config support in containerd.

Start with the OCI image-spec config.md and the referenced runwasi container/context.rs and Spin shim engine.rs. Trace how the entrypoint reaches RuntimeContext and how the shim currently handles it, then determine the expected interaction with the YAML command. Done means a Spin app's OCI entrypoint is handled consistently without breaking the existing container configuration path.