Support cross-repository mounting in Copy

We have already have [Mounter](https://pkg.go.dev/oras.land/oras-go/v2@v2.3.0/registry#Mounter) implemented for [Repository](https://pkg.go.dev/oras.land/oras-go/v2@v2.3.0/registry/remote#Repository.Mount), we can consider leveraging it in [Copy](https://pkg.go.dev/oras.land/oras-go/v2@v2.3.0#Copy) and [CopyGraph](https://pkg.go.dev/oras.land/oras-go/v2@v2.3.0#CopyGraph) for better performance.