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GH-34565: [C++] Teach dataset_writer to accept custom filename functor (#34984)

### Rationale for this change

Existing basename_template will only use a monotonically increasing int as new filenames. when there is needs for custom filenames(left padding, hash-code), downstream users must rename the files in a post-processing step.

### What changes are included in this PR?

A new functor is added to FileSystemDatasetWriteOptions which allows users to provide a custom name for dataset_writer.

### Are these changes tested?

Yes. Unit tests are added for normal and ill-formed lambdas.

### Are there any user-facing changes?

Yes. It allows users to customize output file names.
* Closes: #34565

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