Add append_option support to decimal builders

[ghost]

Which issue does this PR close?

Closes #2224

Rationale for this change

Most of the other builders support append_option which is quite useful for optional fields.

What changes are included in this PR?

Support for append_option in Decimal128Builder and Decimal256Builder.

Are there any user-facing changes?

One new public method on Decimal128Builder and Decimal256Builder.

[tustvold]

There appear to be lint failures

[codecov-commenter]

Codecov Report

Merging #2225 (67407e9) into master (588f408) will decrease coverage by 0.20%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master    #2225      +/-   ##
==========================================
- Coverage   82.52%   82.31%   -0.21%     
==========================================
  Files         240      240              
  Lines       62267    62445     +178     
==========================================
+ Hits        51387    51403      +16     
- Misses      10880    11042     +162     

Impacted Files	Coverage Δ
arrow/src/array/builder/decimal_builder.rs	86.27% <90.00%> (+0.14%)	⬆️
object_store/src/aws.rs	0.00% <0.00%> (ø)
object_store/src/gcp.rs	0.00% <0.00%> (ø)
object_store/src/azure.rs	0.00% <0.00%> (ø)
parquet_derive/src/parquet_field.rs	66.21% <0.00%> (+0.22%)	⬆️
arrow/src/datatypes/datatype.rs	63.00% <0.00%> (+0.31%)	⬆️

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[ghost]

Thanks, should be fixed now @tustvold

[ursabot]

Benchmark runs are scheduled for baseline = 41d96b2 and contender = 561b14c. 561b14c 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