v1.ImageIndex needs a Blob(Hash) method

Given that a descriptor can point to arbitrary content, we need a way to access that content if it doesn't conform to expected media-types. [`layout.layoutIndex`](https://github.com/google/go-containerregistry/blob/6a6302569e968bb450a96bc80af791aa689d53a3/pkg/v1/layout/index.go#L125-L127) already implements this.

We may want to have an option `v1.ImageIndex.Layer(Hash)` as well (and use it for known layer media types), because of how [`MountableLayer`](https://github.com/google/go-containerregistry/blob/6a6302569e968bb450a96bc80af791aa689d53a3/pkg/v1/remote/mount.go#L25) works.

We also need to update a lot of places where we handle `v1.ImageIndex`. A few that spring to mind...

In `mutate`, you can't append non-manifests to an index:
https://github.com/google/go-containerregistry/blob/6a6302569e968bb450a96bc80af791aa689d53a3/pkg/v1/mutate/index.go#L104-L106

In `remote.Write`, we just ignore non-manifests:
https://github.com/google/go-containerregistry/blob/6a6302569e968bb450a96bc80af791aa689d53a3/pkg/v1/remote/write.go#L517-L535

In `remote.MultiWrite`, we return an error:
https://github.com/google/go-containerregistry/blob/6a6302569e968bb450a96bc80af791aa689d53a3/pkg/v1/remote/multi_write.go#L199

In `layout.Write`, we ignore non-manifests: https://github.com/google/go-containerregistry/blob/6a6302569e968bb450a96bc80af791aa689d53a3/pkg/v1/layout/write.go#L261-L263

In `validate.Index`, we log a warning: https://github.com/google/go-containerregistry/blob/6a6302569e968bb450a96bc80af791aa689d53a3/pkg/v1/validate/index.go#L78-L79

In `compare.Indexes`, we don't bother: https://github.com/google/go-containerregistry/blob/6a6302569e968bb450a96bc80af791aa689d53a3/pkg/internal/compare/index.go#L76

Start with pkg/v1/layout/index.go to compare layoutIndex's content access, then inspect the ImageIndex handling in pkg/v1/mutate/index.go, pkg/v1/remote/write.go, pkg/v1/remote/multi_write.go, pkg/v1/layout/write.go, pkg/v1/validate/index.go, and pkg/internal/compare/index.go. First resolve the Blob-versus-Layer API scope; done means the chosen behavior is implemented consistently across these paths.

Written by the indexing model from the issue text.