Made importing from FFI unsafe

[jorgecarleitao]

Using FFI is an intrinsically unsafe operation. However, our public API was marked as safe. This PR changes this.

[codecov]

Codecov Report

Merging #458 (8d3554e) into main (90887c2) will decrease coverage by 0.00%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##             main     #458      +/-   ##
==========================================
- Coverage   80.01%   80.00%   -0.01%     
==========================================
  Files         371      371              
  Lines       22833    22833              
==========================================
- Hits        18269    18267       -2     
- Misses       4564     4566       +2     

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/ffi.rs	68.42% <ø> (ø)
src/array/struct_.rs	35.48% <0.00%> (ø)
src/array/union/ffi.rs	0.00% <0.00%> (ø)
src/array/binary/ffi.rs	84.61% <100.00%> (ø)
src/array/dictionary/ffi.rs	75.00% <100.00%> (ø)
src/array/list/ffi.rs	80.00% <100.00%> (ø)
src/array/primitive/ffi.rs	75.00% <100.00%> (ø)
src/array/utf8/ffi.rs	77.77% <100.00%> (ø)
... and 7 more

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