Use go-eth2-client's IsInfinity() to check relay pubkey

[jtraglia]

📝 Summary

Just fixing a few things I noticed while reviewing.

• Use go-eth2-client's IsInfinity function to check the relay pubkey.
• Provide valid pubkey to TestParseRelaysURLs & TestHexToPublicKey tests.
• Add additional test cases to TestParseRelaysURLs.
• Uncomment "invalid hexadecimal input" tests, might as well.
• Update go-boost-utils to v1.8.1 and run go mod tidy.

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✅ I have run these commands

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 52.56%. Comparing base (d263722) to head (f5924f3).

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #630      +/-   ##
===========================================
+ Coverage    52.37%   52.56%   +0.18%     
===========================================
  Files           14       14              
  Lines         1600     1600              
===========================================
+ Hits           838      841       +3     
+ Misses         706      702       -4     
- Partials        56       57       +1     

Flag	Coverage Δ
unittests	52.56% <100.00%> (+0.18%)	:arrow_up:

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