Add OCI namespace mappings to registry metadata (well-known URL)

Warg has a mechanism for registries to advertise their own preferred namespace->registry mappings for federation purposes. We'd like to introduce the same for OCI-based package registries.

The simplest way I see to do this immediately would be to add this mapping to the `/.well-known/wasm-pkg/registry.json`'s `"oci"` config section. A future possibility - if desired - could be to publish this mapping as an OCI artifact under some reference that doesn't collide with packages.