Add a method into Period class to export a cron expression

[fecaletti]

Is your feature request related to a problem? Please describe.
No, it is a new feature to enable users to use this package with other cron-job related solutions like: https://github.com/agilord/cron

Describe the solution you'd like
I want to propose this package to have a method to export a cron expression for the respective period, in string type, so that we can create period jobs with cron-job packages directly from the created periods.

I think it would be really useful for scheduler applications.

Describe alternatives you've considered
Adding a public method into Period or PeriodGenerator class.
Create a Converter class to convert a Period into specific supported patterns, where one could be Cron Expression.

Additional context
None.

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