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GH-39292 [C++][FS]: Remove the AzureBackend enum and add more flexible connection options (#39293)

### Rationale for this change

It's good to avoid mentioning the specific test environment in the implementation code.

### What changes are included in this PR?

 - Removal of the enum
 - Removal of the `AzureOptions::backend` class member
 - Addition of more options to `AzureOptions`
 - Removal of some private string members of `AzureOptions` -- the URLs are built on-the-fly when the clients are instantiated now

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Changes to the public interface (`azurefs.h`) that won't affect users because the `AzureFS` implementation is not used yet.
* Closes: #39292

Authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Signed-off-by: Felipe Oliveira Carvalho <felipekde@gmail.com>