How could pipelines modify/extend themselves?

Problem Statement

There are certain kinds of pipelines that truly need dynamic behavior in order to be implemented in an efficient way. For example, an operation (DAST & SAST are good examples) where a previously unknown number of jobs should be spawned in order to maximize parallelization. We can solve this by making it possible for a job to spawn other jobs in the same pipeline, resulting in a dynamic CI/CD pipeline. But what is the real problem here and how best to solve it exactly?

See also #21489 (closed)

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