[C++] Remove the AzureOptions::backend option

### Describe the enhancement requested

By adding a more flexible way (URL-based) of determining the backend to use.

**Related Issues:**
- https://github.com/apache/arrow/issues/18014 (is a child of)

### Component(s)

C++