Add randomString() to AuthUtils

[raghavbhatia336]

A small pending task to add duplicate function randomString() to util.js and use it in digest.js and hawk.js.
Todo comment

[codecov]

Codecov Report

Merging #1141 (3acc2d3) into develop (daf1da0) will increase coverage by 0.21%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1141      +/-   ##
===========================================
+ Coverage    90.98%   91.19%   +0.21%     
===========================================
  Files           42       43       +1     
  Lines         2573     2568       -5     
  Branches       741      740       -1     
===========================================
+ Hits          2341     2342       +1     
+ Misses         232      226       -6     

Flag	Coverage Δ
integration	79.75% <100.00%> (+0.19%)	⬆️
legacy	56.15% <93.33%> (+0.14%)	⬆️
unit	49.29% <46.66%> (+0.13%)	⬆️

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Impacted Files	Coverage Δ
lib/authorizer/digest.js	89.89% <100.00%> (-0.37%)	⬇️
lib/authorizer/hawk.js	73.52% <100.00%> (+5.10%)	⬆️
lib/authorizer/util.js	100.00% <100.00%> (ø)

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