Introduced UnionMode enum

[simonvandel]

I feel like seeing UnionMode::Sparse is easier than true

[jorgecarleitao]

I agree. Looks great. Thanks!

[codecov]

Codecov Report

Merging #557 (bbb7c9c) into main (5fc843d) will increase coverage by 0.00%.
The diff coverage is 80.00%.

@@           Coverage Diff           @@
##             main     #557   +/-   ##
=======================================
  Coverage   78.92%   78.93%           
=======================================
  Files         382      382           
  Lines       24276    24291   +15     
=======================================
+ Hits        19160    19173   +13     
- Misses       5116     5118    +2     

Impacted Files	Coverage Δ
src/ffi/schema.rs	65.15% <0.00%> (ø)
src/io/avro/read/schema.rs	52.38% <0.00%> (ø)
src/io/ipc/read/array/union.rs	76.74% <50.00%> (+1.74%)	⬆️
src/datatypes/mod.rs	77.33% <75.00%> (-0.28%)	⬇️
src/array/union/mod.rs	79.41% <88.88%> (-0.20%)	⬇️
src/io/ipc/convert.rs	92.46% <100.00%> (ø)
src/io/json_integration/schema.rs	46.05% <100.00%> (ø)
tests/it/array/mod.rs	100.00% <100.00%> (ø)
tests/it/array/union.rs	100.00% <100.00%> (ø)
tests/it/io/print.rs	100.00% <100.00%> (ø)
... and 3 more

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