[code health] refactor payload attrs checks in handleSubmitNewBlock

[michaelneuder]

📝 Summary

Pull the fee recipient check into an independent function. Follows up on #479.

⛱ Motivation and Context

See M&C of #479.

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✅ I have run these commands

• make lint
• make test-race
• go mod tidy
• I have seen and agree to CONTRIBUTING.md

[codecov-commenter]

Codecov Report

Merging #491 (65e1417) into main (f729e37) will increase coverage by 0.54%.
The diff coverage is 96.77%.

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@@            Coverage Diff             @@
##             main     #491      +/-   ##
==========================================
+ Coverage   30.67%   31.21%   +0.54%     
==========================================
  Files          24       24              
  Lines        4815     4821       +6     
==========================================
+ Hits         1477     1505      +28     
+ Misses       3140     3122      -18     
+ Partials      198      194       -4     

Flag	Coverage Δ
unittests	31.21% <96.77%> (+0.54%)	⬆️

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Files Changed	Coverage Δ
services/api/service.go	36.16% <96.77%> (+1.34%)	⬆️

... and 1 file with indirect coverage changes