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GH-34210: [C++] Make casting timestamp and duration zero-copy when TimeUnit matches (#34270)

### Rationale for this change

Casting from e.g. `timestamp(s, "UTC")` to `timestamp(s)` could be a metadata only change, but is currently a multiplication operation.

### What changes are included in this PR?

This change adds a zero-copy casting path for durations that have equal units and timestamps that have equal units and potentially different timezones.

### Are these changes tested?

We test for correctness and zero-copy.

### Are there any user-facing changes?

No.
* Closes: #34210

Authored-by: Rok Mihevc <rok@mihevc.org>
Signed-off-by: Rok Mihevc <rok@mihevc.org>