* Introduce Volume sub-interface
The Volume interface defines APIs to access a storage provider's
volume (disk) capacity, usage, etc.. It is inherited by the Storage
interface, and is at the same hierarchical level as the Reader
interface.
* Add validations for new Volume method:
Check that GetCapacity() either returns `ErrNotAVolume`, or that it
returns a Capacity struct with values that make sense.
* Implement default (passthrough) GetCapacity:
Cloud providers do not have finite volumes, and WebDAV volumes have no
notion of volume size and usage. These implementations should just
return an error (ErrNotAVolume) when their GetCapacity() is called.
* Implement GetCapacity for sftp storage: Uses the sftp.Client interface
* Implement GetCapacity for logging, readonly store
* Implement GetCapacity() for blobtesting implementations
* Implement GetCapacity() for Google Drive:
Also modifies GetDriveClient to return the entire service instead of just the Files client.
* Implemented GetCapacity() for filesystem storage:
Implemented the function in a seperate file for each OS/architecture (Unix, OpenBSD, Windows).
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