Add form builder

[sashabaranov]

Form builder would enable us to cover image.go, audio.go, and files.go with tests

[codecov]

Codecov Report

Merging #235 (6ea1499) into master (2f3700f) will increase coverage by 8.68%.
The diff coverage is 82.69%.

@@            Coverage Diff             @@
##           master     #235      +/-   ##
==========================================
+ Coverage   73.44%   82.12%   +8.68%     
==========================================
  Files          21       22       +1     
  Lines         625      621       -4     
==========================================
+ Hits          459      510      +51     
+ Misses        125       89      -36     
+ Partials       41       22      -19     

Impacted Files	Coverage Δ
config.go	92.30% <0.00%> (-7.70%)	⬇️
form_builder.go	70.00% <70.00%> (ø)
audio.go	83.01% <90.00%> (+35.72%)	⬆️
client.go	74.07% <100.00%> (ø)
image.go	92.00% <100.00%> (+39.56%)	⬆️

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