Introduce an unexported typeDecoder interface to bsoncodec:

type typeDecoder interface {
    decodeType(DecodeContext, bsonrw.ValueReader) (reflect.Value, error)
}

Also add a decodeAdapter type that contains both a typeDecoder and ValueDecoder which can be added to a Registry.

Modify EmptyInterfaceCodec to implement both typeDecoder and ValueDecoder. When delegating to another codec, it should delegate to typeDecoder if possible and fallback to ValueDecoder if not.

Add a dedicated bson.D decoder which delegates to the typeDecoder for interface{} if it exists (which it should in the default case) and falls back to ValueDecoder if not (e.g. if the decoder for interface{} has been overriden).