Add flexible Duration value parsing in scheduled tasks

[ilyavy]

Closes gh-22013

Add flexible Duration value parsing in scheduled tasks like in Spring Boot for configuration's consistency.

Reasoning about decisions:

• The solution is based on a corresponding commit in Spring Boot
• The logic for parsing delay is not extracted into a separate class following the decision made before by implementors of ScheduledAnnotationBeanPostProcessor class
• However, it is extracted into a separate method, because now it became more complex, and in my opinion, separating it in a different method simplifies the code
• Added method parseDelayAsLong is not documented using JavaDoc because as observed from the code private methods are not documented in Spring

[mrpiggi]

Is there any news on this? Will this feature be integrated into the framework or not?

[snicoll]

Thanks for the PR but we prefer to introduce parsing of a Duration as a core support that can be leveraged elsewhere, see #22474