TypeURL behavior is inconsistent with MarshalAny

[jsternberg]

The TypeURL function will return a typeurl that was either registered with this module or with protobuf. When you obtain the TypeURL, the priority of the choices goes:

• github.com/containerd/typeurl/v2
• google.golang.org/protobuf/proto
• github.com/gogo/protobuf/proto

On the other hand, the MarshalAny function has a different behavior that's inconsistent with this lookup order. Instead, it has the following lookup order:

• google.golang.org/protobuf/proto
• github.com/gogo/protobuf/proto
• github.com/containerd/typeurl/v2

Even though it has this lookup order internally, it uses TypeURL to fill in the typeurl for the any protobuf message. This causes inconsistent behavior when a type implements multiple methods of serialization. If a type has been registered with this library using Register and it is also a proto.Message, it will be given the typeurl that was registered, but it will be serialized with the protobuf format. When the other side tries to deserialize the message, it will get confused and use the wrong deserialization method.

This behavior is accidentally relied upon in buildkit. In buildkit, the message is serialized by using TypeURL and then manually invoking json.Marshal here instead of using MarshalAny. But, it is deserialized with UnmarshalAny. If you change the serialization to use MarshalAny it becomes broken because it uses the wrong TypeURL.

I am not sure what the correct order should be. My thought is that this package should prefer protobuf over the JSON representation, but as long as it is consistent, then I think it's fine. If the order is changed so protobuf serialization is preferred for TypeURL, buildkit will also need to be updated.

[dmcgowan]

// Register a type with a base URL for JSON marshaling.

We should probably prefer JSON if the type is manually registered, otherwise proto can already be preferred by just not registering it with typeurl