Add as_decimal_array function

[liukun4515]

Which issue does this PR close?

Closes #1312

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #1356 (28322d3) into master (a69f0d1) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1356      +/-   ##
==========================================
- Coverage   83.04%   83.03%   -0.01%     
==========================================
  Files         181      181              
  Lines       52925    52930       +5     
==========================================
+ Hits        43950    43952       +2     
- Misses       8975     8978       +3     

Impacted Files	Coverage Δ
arrow/src/array/mod.rs	100.00% <ø> (ø)
arrow/src/array/cast.rs	93.10% <100.00%> (+1.43%)	⬆️
parquet_derive/src/parquet_field.rs	65.98% <0.00%> (-0.46%)	⬇️
arrow/src/datatypes/datatype.rs	66.40% <0.00%> (-0.40%)	⬇️

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[alamb]

❤️