feat: add form support

[DanielRamosAcosta]

This PR adds support for sending multipart/form-data with the following API:

.sendForm({
  name: "Peter",
  info: {
    surname: "Parker",
  },
  avatar: fs.createReadStream("avatar.png"),
  gallery: [
    fs.createReadStream("img1.png"),
    fs.createReadStream("img2.png"),
    fs.createReadStream("img3.png"),
  ],
  other: undefined // these wont be sent
})

[codecov-commenter]

Codecov Report

Merging #95 (7d15181) into main (cce0d07) will increase coverage by 1.96%.
The diff coverage is 97.61%.

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
+ Coverage   94.30%   96.27%   +1.96%     
==========================================
  Files           7        9       +2     
  Lines         123      161      +38     
  Branches       29       34       +5     
==========================================
+ Hits          116      155      +39     
  Misses          5        5              
+ Partials        2        1       -1     

Impacted Files	Coverage Δ
src/tepper.ts	100.00% <ø> (ø)
src/TepperRunner.ts	98.30% <91.66%> (+2.22%)	⬆️
src/TepperBuilder.ts	93.18% <100.00%> (+0.32%)	⬆️
src/utils/objectToFormData.ts	100.00% <100.00%> (ø)
test/utils/expectServerToBeClosed.ts	100.00% <100.00%> (ø)

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[tairosonloa]

Nitpick: may this can be refactored to a recursive function for arrays that have more than one dimension, even I don't think we could need it in a request

[DanielRamosAcosta]

I've been trying to refactor this, but I'm not getting into any conclusion. Maybe a good change for making a refactoring excercise at Acid Labs time