[C++] FixedSizeListBuilder should have UnsafeAppend methods #45723

[cramosme]

Rationale for this change

What changes are included in this PR? I added the three UnsafeAppend methods for FixedSizeListBuilder and replaced

instances in three files to call UnsafeAppend instead of regular append. Files changed were: scalar_nested.cc, fixed_width_test_util.cc, and scalar_string_ascii.cc, along with the header and cc file where functions were declared/defined.

Are these changes tested? Upon each change, I recompiled and rebuilt the program and tested with ctest. Through my testing, everything seemed to be working as usual.

Are there any user-facing changes?

[github-actions]

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