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GH-34644: [C++] Prefer unsafe casting by default in Substrait  (#34645)

### Rationale for this change

See issue

### What changes are included in this PR?

Changes the default cast in Substrait to an unsafe cast.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes

BREAKING CHANGE: The default Substrait behavior has changed slightly.  However, we believe this to be more in line with user expectation.
* Closes: #34644

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