FR: document distribution using OCI indexes

[imjasonh]

Is your feature request related to a problem? Please describe.

The spec states the expectations of a compliant OCI image, but doesn't leave open the possibility that the wasm image may be part of a larger index of images, some potentially other wasm variants (past or future), or images for other platforms entirely (e.g., traditional Linux or Windows images).

Describe the solution you'd like

I'd like the spec to clarify that it accepts indexes, and how it will match image descriptors in the index to find a compliant image to pull and run.

Describe alternatives you've considered

If I have code packaged as both of the two variants of wasm images, I can only distribute them as separate images.

If I have code I've packaged as wasm and as Linux or Windows executables, I can distribute my Linux/Windows images in an index together, but the wasm image has to be distributed separately.

Additional context

https://github.com/opencontainers/image-spec/blob/main/image-index.md