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GH-39339:  [C++] Add ForceCachedHierarchicalNamespaceSupport to help with testing (#39340)

### Rationale for this change

This ensures all the branches in the `AzureFileSystem` code operations are tested.
For instance, many operations executed on a missing container, wouldn't
get a `HNSSupport::kContainerNotFound` error if the cached `HNSSupport` was
already known due to a previous operation that cached the `HNSSupport` value.

### What changes are included in this PR?

Introduction of the helper that overrides `cached_hns_support_` and enumeration of the scenarios.

### Are these changes tested?

Yes. This is a test improvement PR.

* Closes: #39339

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