auth: make sign command accept multiple input files

[alessio]

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[codecov]

Codecov Report

Merging #6208 into master will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #6208      +/-   ##
==========================================
- Coverage   54.75%   54.73%   -0.02%     
==========================================
  Files         447      447              
  Lines       26894    26902       +8     
==========================================
  Hits        14726    14726              
- Misses      11124    11132       +8     
  Partials     1044     1044              

[aaronc]

@alessio would you be able to add a cli_test that reads multiple files?

[alessio]

will do @aaronc

[alessio]

Abandoned in favor of #6350