refactor: improve stitched schema execution

[stuebingerb]

Minor improvement to only compute the usedVariableNames once per request, and not once per remote execution.

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.18%. Comparing base (004f1c8) to head (fc15fa8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...se/kgraphql/schema/structure/RequestInterpreter.kt	50.00%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #444      +/-   ##
==========================================
- Coverage   82.20%   82.18%   -0.02%     
==========================================
  Files         152      152              
  Lines        4776     4778       +2     
  Branches      849      850       +1     
==========================================
+ Hits         3926     3927       +1     
  Misses        533      533              
- Partials      317      318       +1     

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