feat: support fn in headers option

[anshumanv]

• This is a bugfix
• This is a feature
• This is a code refactor
• This is a test update
• This is a docs update
• This is a metadata update

For Bugs and Features; did you add new tests?

yes

Motivation / Use-Case

allow headers function for customizability

Breaking Changes

no

Additional Info

[codecov]

Codecov Report

Merging #3267 (d03b7f7) into master (335a6ed) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3267      +/-   ##
==========================================
+ Coverage   95.38%   95.39%   +0.01%     
==========================================
  Files          34       34              
  Lines        1236     1239       +3     
  Branches      352      353       +1     
==========================================
+ Hits         1179     1182       +3     
  Misses         51       51              
  Partials        6        6              

Impacted Files	Coverage Δ
lib/Server.js	95.40% <100.00%> (+0.02%)	⬆️

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[alexander-akait]

Please add small test

[anshumanv]

sorry for delay, will do today

[anshumanv]

@alexander-akait done

[alexander-akait]

Feel free to merge when CI will be green