add -i flag to upload a custom fw to a board

[umbynos]

With this enhancement, a custom local firmware can be uploaded to a board using -i or --input-file flag:

./arduino-fwuploader firmware flash -i custom_fw.bin -b arduino:samd:mkr1000 -a /dev/ttyACM0

[codecov-commenter]

Codecov Report

Merging #117 (affb754) into main (9220c33) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #117      +/-   ##
==========================================
- Coverage   11.04%   10.96%   -0.08%     
==========================================
  Files          21       21              
  Lines        1793     1805      +12     
==========================================
  Hits          198      198              
- Misses       1537     1549      +12     
  Partials       58       58              

Flag	Coverage Δ
unit	10.96% <0.00%> (-0.08%)	⬇️

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

Impacted Files	Coverage Δ
cli/firmware/flash.go	0.00% <0.00%> (ø)

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[umbynos]

Needs a rebase after #116 gets merged

[ubidefeo]

tested: works as expected