Bug: auth.AppendScopes does not distinguish between registries

[Wwwsylvia]

When copying an artifact from a remote repository to another, we may code something like below:

// add scope hints
scope := auth.ScopeRepository(dst.Reference.Repository, auth.ActionPull, auth.ActionPush)
ctx = auth.AppendScopes(ctx, scope)
// copy
desc, err := oras.Copy(ctx, src, srcRef, dst, dstRef, opts)

This code snippet adds both pull and push scope hints to ctx for the dst repository in order to decrease the number of auth requests on dst in the later copy operation.
However, auth.AppendScopes does not distinguish scopes between registries, resulting in pull and push scopes being requested for both src and dst repository, which is not safe.