Public key file permissions set to full read access

[MalinAhlberg]

Related issue(s) and PR(s)
This PR closes #410 .

Description

• The key pair created by sda-cli is now:

  $sda-cli createKey keys
  $ls -l
  -rw------- 1 malin malin      255 Sep 19 16:36 keys.sec.pem
  -rw-r--r-- 1 malin malin      115 Sep 19 16:36 keys.pub.pem
  

• A test case is added.

How to test
sda-cli createKey testkeys

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 39.17%. Comparing base (d21a0c9) to head (a88ed96).

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #452   +/-   ##
=======================================
  Coverage   39.17%   39.17%           
=======================================
  Files          11       11           
  Lines        1922     1922           
=======================================
  Hits          753      753           
  Misses       1058     1058           
  Partials      111      111           

Flag	Coverage Δ
unittests	39.17% <100.00%> (ø)

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[nanjiangshu]

Looks great!