storage: Export symbols of apiv2/storagepb

## Is your feature request related to a problem? Please describe.
Previously it was not easily possible to write unit testing against cloud storage bindings. With the new GRPC API this is now possible. Currently though one must fork this repo and export the internal symbols manually.

## Describe the solution you'd like
It would be better to have the generated symbols publicly accessible.

## Describe alternatives you've considered
Since bindings get statically registered upon loading not sure whether there is really any alternative.

## Additional context
Not sure how migration could happen since currently the reflection path is used in registering. Changing this path might lead to other problems?