feat: add support to msgspec for kw_only=True

[indrat]

Adds support to the msgspec output for kw_only=True similar to #2102

Removes default IMPORT_MSGSPEC_STRUCT from DEFAULT_IMPORTS since it's not required.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (957ce04) to head (bc21f6b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #2162   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           38        38           
  Lines         4276      4279    +3     
  Branches       986       987    +1     
=========================================
+ Hits          4276      4279    +3     

Flag	Coverage Δ
unittests	99.67% <100.00%> (+<0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #2162 will not alter performance

_{Comparing indrat:msgspec-kw-only-support (bc21f6b) with main (957ce04)}

Summary

✅ 30 untouched benchmarks