Add an Azure "Client" implementation

The "Client" is the interface which is exposed to 'cbbackupmgr' (and other tools) which allows interactions with object stores. This client handles some of the core logic behind populating/syncing the staging directory, validating multipart uploads are resumable and perform bulk operations against objects. A lot of the logic behind staging/syncing is already broken up into (cloud provider agnostic) functions, however, I imagine there's a fair chunk of work that needs to be done for this MB meaning I will likely break it down into multipart sub-tasks.