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 :white_check_mark:

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

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Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/NBISweden/sda-cli/pull/452/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NBISweden) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/NBISweden/sda-cli/pull/452/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NBISweden) | `39.17% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NBISweden#carryforward-flags-in-the-pull-request-comment) to find out more.

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