storage: Provide a way to resume uploads from another instance of the application

**Is your feature request related to a problem? Please describe.**
I'm working on supporting resumable uploads for my customers. My service scales up as needed to many API servers that pass the data to/from GCS. So if I use a Writer() object on one instance of the application and the upload is severed - I need to be able to save the info to a DB in order to allow the user to resume the upload on a different instance of the same application (chosen by the load balancer).

**Describe the solution you'd like**
I can see several different solutions:
1) Allow users to serialize/deserialize the Writer to JSON.
2) Allow the users to ask for just the necessary info needed to resume (like the session URL?), then allow them to create a writer from that info. Ex:
```
writer.GetResumableURL()
NewResumeWriter(url string)
```

**Describe alternatives you've considered**
Writing the entire upload/resume logic myself via http. But that would mean I'm not using this library at all.