Split functionality into three files

[jtraglia]

📝 Summary

I think "functionality" is a bit too vague. Splitting things up makes sense IMO:

• register_validator.go -- implements registerValidator
• get_header.go -- implements getHeader
• get_payload.go -- implements getPayload

Also does some intermediate refactoring, particularly in getPayload.

✅ I have run these commands

• make lint
• make test-race
• go mod tidy

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 76.68998% with 100 lines in your changes missing coverage. Please review.

Project coverage is 43.67%. Comparing base (74a8ecb) to head (1eaf793).
Report is 41 commits behind head on develop.

Files with missing lines	Patch %	Lines
server/get_payload.go	60.94%	46 Missing and 20 partials ⚠️
server/get_header.go	79.59%	21 Missing and 9 partials ⚠️
server/utils.go	95.08%	3 Missing ⚠️
server/service.go	96.15%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #729      +/-   ##
===========================================
- Coverage    43.84%   43.67%   -0.17%     
===========================================
  Files           15       18       +3     
  Lines         1608     1708     +100     
===========================================
+ Hits           705      746      +41     
- Misses         847      904      +57     
- Partials        56       58       +2     

Flag	Coverage Δ
unittests	43.67% <76.68%> (-0.17%)	⬇️

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[jtraglia]

This is a notable change. I've removed getPayloadResponseIsEmpty (only exists in verifyPayload now). I do not believe it's necessary to check this here but I could be wrong. If the response is empty, verifyPayload will return a nil result and this will respond with an error.

[MariusVanDerWijden]

I think this prevents us from accessing nil fields. I don't know if those are set during the unmarshalling. If the are, I think we can remove this check

[jtraglia]

I'm going to close this & break it up into a few PRs.