Capella decode and fallback to bellatrix

[avalonche]

📝 Summary

More context in #411.

As part of #392 this PR attempts to decode the getPayload call with capella types (from attestant) and then decode with bellatrix types (from go-boost-utils) as a fallback.

⛱ Motivation and Context

📚 References

waiting on attestantio/go-builder-client#2 as attestant types don't have support for capella yet for relay responses.

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✅ I have run these commands

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

[codecov-commenter]

Codecov Report

Merging #431 (8cb9cca) into main (ef4cc85) will increase coverage by 0.46%.
The diff coverage is 63.03%.

@@            Coverage Diff             @@
##             main     #431      +/-   ##
==========================================
+ Coverage   65.60%   66.06%   +0.46%     
==========================================
  Files           7        7              
  Lines         907     1049     +142     
==========================================
+ Hits          595      693      +98     
- Misses        275      309      +34     
- Partials       37       47      +10     

Flag	Coverage Δ
unittests	66.06% <63.03%> (+0.46%)	⬆️

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

Impacted Files	Coverage Δ
server/mock_relay.go	84.90% <55.55%> (-2.03%)	⬇️
server/service.go	75.65% <59.13%> (-1.41%)	⬇️
server/utils.go	75.00% <75.60%> (-0.68%)	⬇️

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[metachris]

lgtm!