Add retries for get payload

[avalonche]

📝 Summary

Resolves issue https://github.com/flashbots/mev-boost/issues/323 allowing getPayload to retry the call on errors, as long as it's below the configured timeout

⛱ Motivation and Context

Improves reliability of the getPayload call

📚 References

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✅ I have run these commands

• [x] make lint
• [x] make test-race
• [x] go mod tidy

[codecov-commenter]

Codecov Report

Merging #340 (dcd75c5) into main (19a5221) will increase coverage by 1.09%. The diff coverage is 92.50%.

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@@            Coverage Diff             @@
##             main     #340      +/-   ##
==========================================
+ Coverage   67.69%   68.79%   +1.09%     
==========================================
  Files           8        8              
  Lines        1226     1266      +40     
==========================================
+ Hits          830      871      +41     
+ Misses        347      345       -2     
- Partials       49       50       +1     

Flag	Coverage Δ
unittests	68.79% <92.50%> (+1.09%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/utils.go	82.46% <89.28%> (+1.51%)	:arrow_up:
server/mock_relay.go	88.44% <100.00%> (+2.57%)	:arrow_up:
server/service.go	75.79% <100.00%> (+0.18%)	:arrow_up:

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