feat: Add cronjob duration example

[Mrtenz]

This adds a new example Snap demonstrating cronjobs with ISO 8601 durations instead of cron expressions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.21%. Comparing base (102e818) to head (2394228).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3461   +/-   ##
=======================================
  Coverage   98.21%   98.21%           
=======================================
  Files         404      404           
  Lines       11334    11334           
  Branches     1766     1766           
=======================================
  Hits        11132    11132           
  Misses        202      202           

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