Make streaming layer support explicit

Chatting with @mattmoor -- we [currently try to upload as a streaming layer in remote if `Digest()` fails](https://github.com/google/go-containerregistry/blob/8b5370a4d663c2fd277022f672427f40fe2a26c2/pkg/v1/remote/write.go#L85-L90), we should define a `v1` error to indicate that the layer is a single-pass implementation that needs to be handled differently, then document which packages support this and the ordering of how they will be called.