* cel: fix validation of expression result type
The earlier code used the proto.Equals from github.com/gogo/protobuf, which failed to compare two messages of the same type for some reason. Switching to proto.Equal from the canonical github.com/golang/protobuf fixes the issue.
* deps: remove deprecated github.com/golang/protobuf in favor of google.golang.org/protobuf
* downgrade github.com/smallstep/nosql to resolve warning pb.proto warning