Upgrade getHeader for capella with fallback to bellatrix encoding

[avalonche]

📝 Summary

⛱ Motivation and Context

📚 References

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✅ I have run these commands

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

[codecov-commenter]

Codecov Report

Merging #442 (ff22e15) into main (cbea39d) will increase coverage by 1.40%.
The diff coverage is 79.31%.

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@@            Coverage Diff             @@
##             main     #442      +/-   ##
==========================================
+ Coverage   65.39%   66.80%   +1.40%     
==========================================
  Files           7        8       +1     
  Lines        1049     1211     +162     
==========================================
+ Hits          686      809     +123     
- Misses        315      353      +38     
- Partials       48       49       +1     

Flag	Coverage Δ
unittests	66.80% <79.31%> (+1.40%)	⬆️

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

Impacted Files	Coverage Δ
server/utils.go	75.00% <ø> (ø)
server/types.go	70.00% <70.00%> (ø)
server/mock_relay.go	85.86% <94.23%> (+0.95%)	⬆️
server/service.go	74.90% <100.00%> (+0.56%)	⬆️

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