[code health] Refactor fee recipient check in handleSubmitNewBlock

[michaelneuder]

📝 Summary

Pull the fee recipient check into an independent function. This is the first PR in a set to refactor the handleSubmitNewBlock function to be more modular. This increases readability and unit-testability of the components that make up a block submission, and allows for this logic to be reused in other block submission paths.

⛱ Motivation and Context

In order to make #466 easier to review, we want to refactor some of the block submission path to be reusable for v2 optimistic submissions. This has the nice added benefit of improving the code health of the relay as a whole, which is a nice perk :-)

📚 References

https://notes.ethereum.org/@mikeneuder/optimistic-v2 describes the status of v2 optimistic relaying.

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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 #479 (3922f05) into main (372dcd1) will increase coverage by 0.26%.
The diff coverage is 95.00%.

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@@            Coverage Diff             @@
##             main     #479      +/-   ##
==========================================
+ Coverage   30.58%   30.84%   +0.26%     
==========================================
  Files          24       24              
  Lines        4783     4788       +5     
==========================================
+ Hits         1463     1477      +14     
+ Misses       3121     3113       -8     
+ Partials      199      198       -1     

Flag	Coverage Δ
unittests	30.84% <95.00%> (+0.26%)	⬆️

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Impacted Files	Coverage Δ
services/api/service.go	34.94% <95.00%> (+0.72%)	⬆️