reject prysm getHeader

[michaelneuder]

📝 Summary

During capella the relays see failures in prysm signatures with the error:

could not verify capella payload signature

This PR rejects getHeader requests from prysm clients based on the UserAgent.

✅ 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 #351 (96ecbda) into main (2d2bdc0) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

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@@            Coverage Diff             @@
##             main     #351      +/-   ##
==========================================
- Coverage   17.73%   17.71%   -0.03%     
==========================================
  Files          20       20              
  Lines        3698     3703       +5     
==========================================
  Hits          656      656              
- Misses       2959     2964       +5     
  Partials       83       83              

Flag	Coverage Δ
unittests	17.71% <0.00%> (-0.03%)	⬇️

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

Impacted Files	Coverage Δ
services/api/service.go	13.06% <0.00%> (-0.06%)	⬇️

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[ihooni]

Prysm is now fixing this issue

OffchainLabs/prysm#12263