Add String to CronExpression conversion support

[amiriahmad72]

Add String to CronExpression conversion support.

This converter use CronExpression parse () method to convert String to CronExpression.

For example, it helps us to use type-safe CronExpression in ConfigurationProperties classes.

[wilkinsona]

Thanks for the suggestion. Given that CronExpression is a Spring Framework type, I wonder if the should be added to Spring Framework rather than to Spring Boot. I'll discuss it with the Framework team.

[wilkinsona]

@amiriahmad72 it would also be useful to know how you're using CronExpression. There's very little, if any, API in Spring Framework that uses it. For example, CronTrigger takes a String rather than a pre-parsed CronExpression instance. It's not yet clear to me how having such conversion support would be generally useful.