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GH-39345: [C++][FS][Azure] Add support for environment credential (#41715)



### Rationale for this change
Maybe be useful to support explicit environment credential (currently environment credential can be used as part of the Azure default credential flow). 

### What changes are included in this PR?

### Are these changes tested?
There are new unittests but no integration tests that we can actually authenticate successfully. We are relying on the Azure C++ SDK to abstracting that away. 

### Are there any user-facing changes?
Yes, environment credential is now available. 

* GitHub Issue: #39345

Authored-by: Thomas Newton <thomas.w.newton@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>