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GH-34386: [C++] Add a PathFromUriOrPath method (#34420)

### Rationale for this change

We have some URI parsing indirectly exposed through FilesystemFromUri.  However, this isn't very useful if the user has multiple URIs or if they already have a filesystem.  This method allows that same URI handling to be used even if the user already has a filesystem.

### What changes are included in this PR?

Adds a new arrow::fs::PathFromUriOrPath method

### Are these changes tested?

Yes, via unit tests

### Are there any user-facing changes?

There is a new API method but no changes to any existing APIs
* Closes: #34386

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>