Removed Option from fields' metadata

[jorgecarleitao]

Thereby simplifying its API

[codecov]

Codecov Report

Merging #715 (7f973fe) into main (d14ae86) will increase coverage by 0.01%.
The diff coverage is 64.58%.

@@            Coverage Diff             @@
##             main     #715      +/-   ##
==========================================
+ Coverage   70.59%   70.60%   +0.01%     
==========================================
  Files         312      312              
  Lines       16922    16892      -30     
==========================================
- Hits        11946    11927      -19     
+ Misses       4976     4965      -11     

Impacted Files	Coverage Δ
src/io/json_integration/write/schema.rs	0.00% <0.00%> (ø)
src/datatypes/field.rs	27.45% <37.50%> (-12.83%)	⬇️
src/io/avro/read/schema.rs	52.68% <66.66%> (+0.05%)	⬆️
src/io/json_integration/read/schema.rs	64.00% <80.00%> (ø)
src/datatypes/mod.rs	81.39% <100.00%> (-0.43%)	⬇️
src/ffi/schema.rs	68.47% <100.00%> (ø)
src/io/ipc/read/schema.rs	90.42% <100.00%> (+0.42%)	⬆️
src/io/ipc/write/schema.rs	92.77% <100.00%> (-0.06%)	⬇️
src/compute/arithmetics/time.rs	26.60% <0.00%> (+0.91%)	⬆️
... and 1 more

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