Add UnionArray

[jorgecarleitao]

• Added UnionArray
  • equality
  • Display
  • read and write IPC (V4 and V5)
  • write to string (io::print::write)
  • produce and consume FFI
  • UnionArray::iter() with an iterator over values via the Scalar API
  • memory estimation (compute::aggregate::memory)
• Added roundtrip tests against IPC
• changed IPC read APIs to support different IPC versions (Union validity changed from V4 to V5 and reading it requires knowing which version the file was written as) (Add MetadataVersion to IPC interfaces #282)
• changed DataType::Union to contain necessary metadata for IPC and FFI (Change DataType::Union to enable round trips in IPC #281)

I verified that integration tests against C++ pass when union is not skipped.

Closes #281 and #282

[codecov]

Codecov Report

Merging #283 (3fc6c9d) into main (8237800) will decrease coverage by 0.02%.
The diff coverage is 64.78%.

@@            Coverage Diff             @@
##             main     #283      +/-   ##
==========================================
- Coverage   77.58%   77.55%   -0.03%     
==========================================
  Files         255      260       +5     
  Lines       20709    21048     +339     
==========================================
+ Hits        16067    16324     +257     
- Misses       4642     4724      +82     

Impacted Files	Coverage Δ
arrow-flight/src/utils.rs	0.00% <ø> (ø)
...ng/src/flight_server_scenarios/integration_test.rs	0.00% <ø> (ø)
src/array/ffi.rs	42.10% <0.00%> (-1.14%)	⬇️
src/array/growable/mod.rs	36.23% <ø> (ø)
src/array/union/ffi.rs	0.00% <0.00%> (ø)
src/compute/aggregate/memory.rs	27.69% <0.00%> (-3.35%)	⬇️
src/datatypes/field.rs	18.64% <0.00%> (ø)
src/datatypes/mod.rs	11.76% <ø> (ø)
src/ffi/array.rs	79.20% <0.00%> (-0.80%)	⬇️
src/ffi/schema.rs	55.29% <0.00%> (-6.24%)	⬇️
... and 40 more

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