Add option to disable child pipeline email notifications

What does this MR do and why?

Resolves #587443

Add a configurable checkbox to allow users to opt out of notifications for child pipelines while still receiving parent pipeline notifications.

Defaults to enabled to maintain backward compatibility.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a project
2. Enable Pipeline Status Emails integration for all branches and all statuses
3. Create a CI/CD YAML file in that project that triggers a child pipeline
4. Confirm receipt of email from the parent and child pipelines
5. In the Pipeline Status Emails integration, click the checkbox for "Notify child pipelines"
6. Confirm receipt of email from the parent pipeline only

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