Add deserialization of Bytes -> Decimal

[jaychia]

Arrow2 already has support for Parquet FixedLenByteArray -> Decimal conversion

This PR adds support for Parquet (variable-length) ByteArray -> Decimal conversion, re-using most of the logic from FixedLenByteArray conversion

[ritchie46]

This PR adds support for Parquet (variable-length) ByteArray

I don't understand. Why would decimal be encoded in variable length binary?

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.05% ⚠️

Comparison is base (87ab844) 83.02% compared to head (ab04856) 82.98%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1534      +/-   ##
==========================================
- Coverage   83.02%   82.98%   -0.05%     
==========================================
  Files         391      391              
  Lines       42786    42814      +28     
==========================================
+ Hits        35523    35529       +6     
- Misses       7263     7285      +22     

Files Changed	Coverage Δ
src/io/parquet/read/deserialize/simple.rs	82.73% <0.00%> (-3.54%)	⬇️
src/io/parquet/read/schema/convert.rs	93.73% <0.00%> (-0.49%)	⬇️

... and 6 files with indirect coverage changes

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[jaychia]

Hi @ritchie46, apologies for the late reply!

Going by the Parquet spec, decimals are actually able to be encoded as int32, int64, fixed_len_byte_array and binary.

See: https://github.com/apache/parquet-format/blob/master/LogicalTypes.md#decimal

binary: precision is not limited, but is required. The minimum number of bytes to store the unscaled value should be used.

[ariesdevil]

Also need to impl for nested https://github.com/jorgecarleitao/arrow2/blob/main/src/io/parquet/read/deserialize/nested.rs