add optional roleArn support for scheduler schedules

[jerolan]

The reason behind this PR is that my team and I identified a security issue in allowing a role that was created for a specific purpose to be modified to serve a completely different one, giving whoever assumed that role permissions beyond the minimum they should have had.

Summary

• add optional schedule.roleArn support for method: scheduler
• keep existing behavior as default fallback to the function execution role when roleArn is not provided
• document the new option in schedule event docs
• update TypeScript types for schedule event configuration

Changes

• extend schedule event schema to accept roleArn (string or CloudFormation function)
• set scheduler target RoleArn from schedule.roleArn when present, otherwise fallback to function role
• verifies tests for both paths:
  • explicit scheduler role
  • fallback to function role

[mnapoli]

Thanks!