crypto: add StringCompressed() for PublicKey

[AliceInHunterland]

Close #3263

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.10%. Comparing base (8913542) to head (fc79d38).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3408      +/-   ##
==========================================
+ Coverage   86.08%   86.10%   +0.01%     
==========================================
  Files         331      331              
  Lines       38107    38108       +1     
==========================================
+ Hits        32805    32812       +7     
+ Misses       3785     3781       -4     
+ Partials     1517     1515       -2     

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[AnnaShaleva]

Contribution guideline job is failing.