Added support for Extension (logical) type

[jorgecarleitao]

Adds support for Arrow's Extension "DataType", enabling users to define and use custom data types from Arrow's ecosystem.

Note that, from now on, array's data_type is not sufficient to downcast an array. I have updated the user guide with the new rules. The easiest way to think about them:

• enum PhysicalType has a one-to-one relationship to concrete arrays (e.g. PhysicalType::Utf8 <-> Utf8Array<i32>)
• enum DataType has a many-to-one relationship with a PhysicalType, which can be obtained via .to_physical_type() -> PhysicalType.

The corollary of this change is that, to correctly downcast arrays, users should now use match array.data_type().to_physical_type(). This is only relevant to the ExtensionType, which boxes a DataType which indicates its own logical type.

Thanks to @sundy-li for the initial implementation at #338 and follow-up discussions at #350.

Closes #361

[codecov]

Codecov Report

Merging #359 (e1bd078) into main (145b7c8) will increase coverage by 0.07%.
The diff coverage is 79.49%.

@@            Coverage Diff             @@
##             main     #359      +/-   ##
==========================================
+ Coverage   81.23%   81.30%   +0.07%     
==========================================
  Files         326      326              
  Lines       20969    21000      +31     
==========================================
+ Hits        17035    17075      +40     
+ Misses       3934     3925       -9     

Impacted Files	Coverage Δ
src/array/display.rs	57.89% <ø> (ø)
src/array/fixed_size_list/mod.rs	54.90% <0.00%> (+0.90%)	⬆️
src/array/list/mod.rs	80.00% <0.00%> (-0.89%)	⬇️
src/datatypes/field.rs	19.32% <0.00%> (+0.68%)	⬆️
src/ffi/schema.rs	57.91% <0.00%> (-0.27%)	⬇️
src/array/dictionary/mod.rs	69.76% <50.00%> (+1.58%)	⬆️
src/scalar/mod.rs	61.90% <50.00%> (-4.77%)	⬇️
src/io/json_integration/schema.rs	44.85% <55.73%> (+0.83%)	⬆️
src/array/union/mod.rs	82.65% <69.23%> (+2.24%)	⬆️
src/array/struct_.rs	41.33% <80.00%> (ø)
... and 17 more

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