feat(gen): allow sum types as type discriminator

[tdakkota]

No description provided.

[codecov]

Codecov Report

Merging #300 (a9adcfa) into main (5610658) will decrease coverage by 0.03%.
The diff coverage is 68.18%.

@@            Coverage Diff             @@
##             main     #300      +/-   ##
==========================================
- Coverage   66.91%   66.87%   -0.04%     
==========================================
  Files         117      117              
  Lines        7785     7823      +38     
==========================================
+ Hits         5209     5232      +23     
- Misses       2227     2239      +12     
- Partials      349      352       +3     

Impacted Files	Coverage Δ
gen/schema_gen_sum.go	81.96% <62.50%> (-2.99%)	⬇️
gen/ir/json.go	82.58% <75.00%> (-0.84%)	⬇️

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