feat: msgspec discriminated unions

[indrat]

adds support for:

• adding base class kwargs to the msgpsec template in order to support adding tag and tag_field kwargs for Struct.
• the discriminator property, uses the propertyName as the tag_field and the literal value as the tag.

this may not be the ideal method for doing this so happy for any feedback to improve it.

aiming to address #1610

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #2081 will not alter performance

_{Comparing indrat:discriminated-unions-msgspec-support (c56c569) with main (6c3e114)}

Summary

✅ 30 untouched benchmarks

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (6c3e114) to head (c56c569). Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #2081   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           38        38           
  Lines         4263      4273   +10     
  Branches       984       986    +2     
=========================================
+ Hits          4263      4273   +10     

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

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