Added API to FFI Field

[jorgecarleitao]

So that the fields can be imported and exported from the C data interface.

The public API is different since ownership rules changed (#328); see examples/ffi.rs with a complete example.

Closes #328.

[codecov]

Codecov Report

Merging #321 (1fc3e04) into main (3ce4f1c) will increase coverage by 0.08%.
The diff coverage is 69.23%.

@@            Coverage Diff             @@
##             main     #321      +/-   ##
==========================================
+ Coverage   80.53%   80.61%   +0.08%     
==========================================
  Files         324      325       +1     
  Lines       21208    21220      +12     
==========================================
+ Hits        17080    17107      +27     
+ Misses       4128     4113      -15     

Impacted Files	Coverage Δ
arrow-pyarrow-integration-testing/src/lib.rs	0.00% <ø> (ø)
src/array/boolean/ffi.rs	0.00% <0.00%> (ø)
src/array/struct_.rs	42.85% <0.00%> (ø)
src/array/union/ffi.rs	0.00% <0.00%> (ø)
src/ffi/schema.rs	58.18% <44.77%> (+2.88%)	⬆️
src/array/binary/ffi.rs	81.81% <100.00%> (ø)
src/array/list/ffi.rs	80.00% <100.00%> (ø)
src/array/primitive/ffi.rs	75.00% <100.00%> (ø)
src/ffi/array.rs	41.66% <100.00%> (-1.58%)	⬇️
src/ffi/ffi.rs	92.08% <100.00%> (+5.63%)	⬆️
... and 5 more

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[jorgecarleitao]

fwi @ritchie46 , since this adds some backward incompatible changes to FFI.

OTOH, we can now share Field in the FFI

[ritchie46]

Right, will take a look!