SSE Payload Attributes subscription

[avalonche]

📝 Summary

Adds SSE payload attributes subscription on the relay.

⛱ Motivation and Context

As part of ethereum/beacon-APIs#305 SSE endpoint providing payload attributes allows the relay to retrieve prev_randao and withdrawals with a custom BN fork to retrieve this information.

📚 References

---

✅ 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 #318 (32a2e6e) into main (6640511) will decrease coverage by 0.41%.
The diff coverage is 0.00%.

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

@@            Coverage Diff             @@
##             main     #318      +/-   ##
==========================================
- Coverage   18.34%   17.94%   -0.41%     
==========================================
  Files          20       20              
  Lines        3559     3639      +80     
==========================================
  Hits          653      653              
- Misses       2826     2905      +79     
- Partials       80       81       +1     

Flag	Coverage Δ
unittests	17.94% <0.00%> (-0.41%)	⬇️

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

Impacted Files	Coverage Δ
beaconclient/mock_beacon_instance.go	65.71% <0.00%> (-0.96%)	⬇️
beaconclient/multi_beacon_client.go	48.43% <0.00%> (-1.04%)	⬇️
beaconclient/prod_beacon_instance.go	18.32% <0.00%> (-3.31%)	⬇️
services/api/service.go	13.08% <0.00%> (-0.61%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[metachris]

this function will be called multiple times for each payload attributes -- namely once per CL client. therefore this function should somehow guard against it.

question: do we need to support multiple parents? or do we just always use the latest received one?

[avalonche]

makes sense to compare the parent hash as well but I think we can use the hash from the "best" BN from in the multi-BN instance to keep consistent state?

[metachris]

Deployed to Sepolia and Zhejiang testnets!