fix: use signed comparator to compare decimal128 and decimal256

[liukun4515]

Which issue does this PR close?

Closes #2256

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #2275 (f260d58) into master (b4fa47d) will increase coverage by 0.01%.
The diff coverage is 95.31%.

@@            Coverage Diff             @@
##           master    #2275      +/-   ##
==========================================
+ Coverage   82.29%   82.30%   +0.01%     
==========================================
  Files         245      245              
  Lines       62688    62750      +62     
==========================================
+ Hits        51586    51646      +60     
- Misses      11102    11104       +2     

Impacted Files	Coverage Δ
arrow/src/util/decimal.rs	91.14% <95.31%> (+4.22%)	⬆️
arrow/src/datatypes/datatype.rs	62.30% <0.00%> (-0.32%)	⬇️
parquet/src/arrow/schema.rs	96.76% <0.00%> (-0.18%)	⬇️

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[viirya]

Suggested change

//

[ursabot]

Benchmark runs are scheduled for baseline = 58dc611 and contender = ed9fc56. ed9fc56 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java