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GH-39449: [C++] Use default Azure credentials implicitly and support anonymous credentials explicitly (#39450)

### Rationale for this change

 - Default credentials should be used by default.
 - There should be a way to connect to public containers without credentials (aka "anonymous credential").

### What changes are included in this PR?

 - Sync ordering of declarations and definitions in the `AzureOptions` classs
 - Use default credentials even when `ConfigureDefaultCredential()` isn't explicitly called
 - Create clients when `credential_kind_` is "anonymous" instead of returning an error
 
### Are these changes tested?

By new and existing tests.
* Closes: #39449

Lead-authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Felipe Oliveira Carvalho <felipekde@gmail.com>