Implement allowing child pipeline to have child pipeline

What does this MR do?

Related to: #29651 (closed)

In this MR, we are just allowing parent->child->child nesting. This is controlled by MAX_DESCENDANTS_DEPTH constant of Ci::CreateCrossProjectPipelineService class.

This feature is also behind a FF ci_child_of_child_pipeline for now. (#243747 (closed))

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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