feat: introduce toduration conversion function

[42atomys]

Description

This pull request introduce the toDuration as proposed in #8 and Masterminds/sprig#388 using the cast package.

Changes

• Add the function toDuration

Fixes #8

Checklist

• I have read the CONTRIBUTING.md document.
• My code follows the code style of this project.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the documentation accordingly.
• This change requires a change to the documentation on the website.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.7%. Comparing base (f253487) to head (75537b4).

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #27   +/-   ##
=====================================
  Coverage   99.7%   99.7%           
=====================================
  Files         13      13           
  Lines       1398    1401    +3     
=====================================
+ Hits        1394    1397    +3     
  Misses         2       2           
  Partials       2       2           

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