GH-35815: [C++][Compute] Support dictionary inputs in UTF-8 trim

[LarryHu0217]

Rationale for this change

The UTF-8 trim functions only registered kernels for string and large-string inputs. Their exact dispatch therefore rejected dictionary-encoded strings even though the dictionary value type was supported.

What changes are included in this PR?

• Allow the stateful unary string-kernel helper to use a specialized function type.
• Decode dictionary input types during dispatch for utf8_trim, utf8_ltrim, and utf8_rtrim.
• Add regression coverage for dictionary-encoded string and large-string arrays, including nulls.

Are these changes tested?

• arrow-compute-scalar-type-test --gtest_filter='*TrimUTF8*' (4 passed)
• arrow-compute-scalar-type-test --gtest_brief=1 (344 passed, 1 pre-existing disabled test)
• C++ Debug build with warnings treated as errors
• clang-format 18.1.8 and cpplint 1.6.1

Are there any user-facing changes?

Yes. The three UTF-8 trim functions now accept dictionary-encoded string inputs and return decoded string arrays. There are no public API signature changes.

AI-assisted development

OpenAI Codex assisted with reproducing the issue, implementing the patch, and drafting the regression tests. The change was checked against Arrow's existing dictionary-dispatch patterns and validated with the commands above.

• GitHub Issue: [C++][Compute] Function 'utf8_trim' has no kernel matching input types (dictionary<values=string, indices=int64, ordered=0>) #35815

[github-actions]

⚠️ GitHub issue #35815 has been automatically assigned in GitHub to PR creator.