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GH-39262: [C++][Azure][FS] Add default credential auth configuration (#39263)



### Rationale for this change
Default credential is a useful auth option. 

### What changes are included in this PR?
Implement `AzureOptions::ConfigureDefaultCredential` plus a little bit of plumbing to go around it. 
Created a simple test. 

### Are these changes tested?
Added a simple unittest that everything initialises happily. This does not actually test a successful authentication. I think to do a real authentication with Azure we would need to run the test against real blob storage and we would need to create various identities which are non-trivial to create. Personally I think this is ok because all the complexity is abstracted away by the Azure SDK. 

### Are there any user-facing changes?

* Closes: #39262

Lead-authored-by: Thomas Newton <thomas.w.newton@gmail.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>