Ensure that fork version is 4 bytes

[jtraglia]

📝 Summary

If a custom genesis fork version that's fewer than 4 bytes (e.g., -genesis-fork-version 0xabcd) is specified, the missing bytes will be filled with "random" bytes (e.g., 0xabcd6364). This PR will return an error if the fork version isn't exactly 4 bytes. Previously, it only returned an error if it was greater than 4 bytes.

⛱ Motivation and Context

Fixes #277.

✅ I have run these commands

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

[codecov-commenter]

Codecov Report

Merging #395 (2fbac58) into main (f146349) will decrease coverage by 0.33%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #395      +/-   ##
==========================================
- Coverage   66.07%   65.74%   -0.34%     
==========================================
  Files           7        7              
  Lines         902      902              
==========================================
- Hits          596      593       -3     
- Misses        271      273       +2     
- Partials       35       36       +1     

Flag	Coverage Δ
unittests	65.74% <0.00%> (-0.34%)	⬇️

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

Impacted Files	Coverage Δ
server/utils.go	75.67% <0.00%> (ø)
server/service.go	76.95% <0.00%> (-0.70%)	⬇️

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