Add support for SAS Keys for Azure Blob

[pjbgf]

Azure Blob supports a wide range of authentication methods. Flux supports all of them, with the exception of SAS Keys.

SAS Keys are a series of information, including a SAS Token, which are defined as query string parameters in the storage resource URI and are used by Azure for authenticating the request.
Source controller should add support for copying such information from secrets and injecting into the base URL on-demand, just before sending it over to the Azure SDK.