Feat/add custom headers

[DanielRamosAcosta]

This PR adds support for adding custom headers with the following API:

await tepper(app)
  .get("/")
  .withHeaders({
    "X-Custom-Header": "custom-header-value",
  })
  .run()

[codecov-commenter]

Codecov Report

Merging #141 (4b9c095) into main (88d9141) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #141      +/-   ##
==========================================
+ Coverage   96.04%   96.08%   +0.04%     
==========================================
  Files          10       10              
  Lines         177      179       +2     
  Branches       40       40              
==========================================
+ Hits          170      172       +2     
  Misses          6        6              
  Partials        1        1              

Impacted Files	Coverage Δ
src/TepperRunner.ts	98.33% <ø> (ø)
src/tepper.ts	100.00% <ø> (ø)
src/TepperBuilder.ts	93.47% <100.00%> (+0.29%)	⬆️

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