feat(schedule): add support for groupName

[fbnfgc]

relates to #143

Summary

Allow schedule.groupName when using method: scheduler.

By default, osls omits GroupName, so EventBridge Scheduler uses the AWS default schedule group. This change only opts in when groupName is explicitly configured.

Why

method: scheduler currently always creates AWS::Scheduler::Schedule resources in the default schedule group. That makes it hard to use IAM conditions or ABAC policies scoped to dedicated scheduler groups, and forces projects to either loosen permissions or avoid method: scheduler.

CloudFormation already supports this through the optional GroupName property on AWS::Scheduler::Schedule, so this exposes that capability in osls.

[mnapoli]

Thanks!