Storage: add support for Per-Object Storage Class

[omaray]

We need to add support for Per-Object Storage Class (POSC).

Here some extra information to help:

• POSC info from Cloud Storage documentation
• POSC example with JSON API
• Storage Class info from Cloud Storage documentation

On top of that, google-cloud-dotnet has made an implementation which can be used as reference.

• Please view the Pull Request to understand the changes

If you have any implementation questions, please direct them at jskeet

/cc @jskeet @danoscarmike

[lukesneeringer]

@tseaver Is this something you would be able to take on?

[tseaver]

Wrinkles on the per-object API:

• The storageClass property can only be "set" during object creation.
• "Updating" a blob's storage class cannot be done via PATCH: instead, one must perform a rewrite, passing the storageClass property dict in the body.

To handle this properly, we can't just re-use the Bucket.storage_class setter: we should only allow setting the value if the object does not already exist (e.g., has no createdTime). We will need to provide another API-invoking method, update_storage_class, which performs the POST request to the appropriate rewriteTo URL, passing the new property in the body.

[tseaver]

Because we don't send along blob properties during upload_* operations (#754, #1660), there is no point in adding a storage_class setter at all.

[lukesneeringer]

@tseaver This change is purely additive, correct? I can cut a storage 0.23.1 safely?

[tseaver]

@lukesneeringer Sorry for the lag: yes, it is purely additive.

[lukesneeringer]

@tseaver No worries, and thanks. I will probably cut minor updates to things on Monday morning.

[tseaver]

@lukesneeringer It occurs to me that calling the release 0.23.1 is actually a semver violation: it isn't a bugfix, but a feature addition.

[lukesneeringer]

My understanding is that semver is picky about that post 1.0, but less so in the 0 phase.
I have basically been using the middle-number bump for breaking changes, because we version using >=0.23.x, <0.24, and for these types of changes, my goal is to opt people in automatically.

Open to another approach.

[daspecster]

I talked to @lukesneeringer a tiny bit about this offline. I think we don't have a solid plan of how versioning should work since we split out the services from gcloud. I could be wrong but I think the umbrella package should be incrementing a lot faster and be at a higher number now?