Implementing more 1.3 fields for structures

[Lattay]

Implementing fields added here: Materials-Consortia/OPTIMADE#555

• fractional_site_positions
• optimization_type
• site_coordinate_span
• site_coordinate_span_description

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.41%. Comparing base (57da6be) to head (7576184).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2418      +/-   ##
==========================================
+ Coverage   90.28%   90.41%   +0.12%     
==========================================
  Files          78       78              
  Lines        5086     5110      +24     
==========================================
+ Hits         4592     4620      +28     
+ Misses        494      490       -4     

Flag	Coverage Δ
project	90.41% <100.00%> (+0.12%)	⬆️
validator	90.41% <100.00%> (+0.12%)	⬆️

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[ml-evs]

Thanks for this @Lattay! I switched the optimization_type and site_coordinate_span fields to enums and added an extra couple of tests cases, but otherwise good to go!