Enhance output objects

[munrojm]

This PR introduces post-processing on returned document models to:

• Remove unset attributes and add them to new fields_not_requested attribute
• Add custom __repr__ and __str__ methods
• Add custom attribute error for unset but available data

[codecov-commenter]

Codecov Report

Merging #654 (2b4092d) into main (ef951a2) will increase coverage by 0.31%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #654      +/-   ##
==========================================
+ Coverage   84.74%   85.06%   +0.31%     
==========================================
  Files          36       36              
  Lines        1501     1513      +12     
==========================================
+ Hits         1272     1287      +15     
+ Misses        229      226       -3     

Impacted Files	Coverage Δ
src/mp_api/core/client.py	88.85% <66.66%> (+1.24%)	⬆️

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