Implement CronValidator for KindCacheConfig cronExpr

Description:

This task involves implementing a custom CronValidator to validate the cronExpr field within the Kind class. The validation must support standard cron expressions compatible with Spring's CronExpression. Additionally, unit tests should be added to validate the CRON expressions directly and ensure proper validation logic.

Requirements:

1. Cron Expression Validation:

   • Implement a @ValidCron annotation for validating CRON expressions compatible with Spring's CronExpression.
   • Create a CronExpressionValidator class to handle validation logic, ensuring the CRON expressions conform to the six-field format expected by Spring.

2. API and Services:

   • Ensure that the cronExpr field in the Kind class is annotated with @ValidCron.
   • The cronExpr field should be validated in all API endpoints where Kind objects are created or updated.
   • Return appropriate error messages for invalid CRON expressions.

3. Unit Tests:

   • Add unit tests to test the CronExpressionValidator directly, covering:
     • Valid CRON expressions
     • Invalid CRON expressions
     • Complex CRON expressions
     • Blank or null CRON expressions

4. Swagger Documentation:

   • Update Swagger documentation to include validation rules and examples for the cronExpr field.

Acceptance Criteria:

• The @ValidCron annotation and CronExpressionValidator have been implemented and applied to the cronExpr field in the Kind class.
• Unit tests have been added to directly test the CronExpressionValidator, covering various CRON expression scenarios (valid, invalid, complex, null, blank).
• The API properly validates CRON expressions and returns appropriate error messages.
• Swagger documentation is updated to reflect the validation rules for the cronExpr field.