Add holesky network details and flag

[avalonche]

📝 Summary

This PR adds support for the holesky testnet as the holesky relay at https://boost-relay-holesky.flashbots.net/ is up now.

⛱ Motivation and Context

This provides native support for validators on holesky to test mev-boost and the relay.

📚 References

See holesky testnet for network details.

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✅ I have run these commands

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

[codecov-commenter]

Codecov Report

Merging #585 (ca63208) into develop (4748718) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

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@@             Coverage Diff             @@
##           develop     #585      +/-   ##
===========================================
- Coverage    69.41%   69.39%   -0.03%     
===========================================
  Files            7        7              
  Lines         1102     1111       +9     
===========================================
+ Hits           765      771       +6     
- Misses         289      296       +7     
+ Partials        48       44       -4     

Flag	Coverage Δ
unittests	69.39% <0.00%> (-0.03%)	⬇️

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

Files	Coverage Δ
cli/main.go	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

[metachris]

lgtm!