C.41 compliance: Azure factory constructor.

[bekadavis9]

Remove Azure::init in favor of a C.41-compliant factory constructor. The Azure::azure_params_ member variable is no longer optional and is fully-initialized at construction time.

[sc-60071]

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TYPE: IMPROVEMENT
DESC: C.41 constructor: Azure.

[teo-tsirpanis]

(some of the comments apply to the PRs for GCS and HDFS, I will review them more thorougly after reviewing this PR finishes)

I am not a fan of the ***_within_VFS mechanism and cannot understand why it was introduced. My attempts to find more context in source code documentation, the review comments of #4360 or the Shortcut story were unfruitful. We can't easily remove it for S3 because test code has a dependency on it, but we shouldn't do the same for the other VFSes.

[teo-tsirpanis]

After we do that it might be a good idea to restore the default constructor.

[bekadavis9]

Note that Azure::read and AzureClientSingleton::upload_block cannot yet be un-Status-edited. This work will be done later, when all of VFS receives the same treatment.

[teo-tsirpanis]

Great work with removing Status, thanks!

I left some comments and still believe that Azure_within_VFS should be removed before I approve.

[bekadavis9]

cc: @teo-tsirpanis

After discussion with @ihnorton offline, we've decided to stick with the current design. This entails use of Backend_within_VFS and directly passing a Config into the backends' constructors. A larger refactor can be considered once a full design for VFS rework is fleshed out.

[teo-tsirpanis]

This entails use of Backend_within_VFS and directly passing a Config into the backends' constructors.

We could do these as part of this larger refactor, where there will be more context for understanding their necessity, but I don't mind either way.