Introducing parent/child pipelines

Introduces parent/child pipelines for Bitbucket Pipelines, allowing a step in one pipeline to trigger and encapsulate another custom pipeline. This enables greater parallelism, modularity, and seamless reruns, addressing challenges with monolithic pipeline configurations and the 100-step limit. New step properties 'type: pipeline' and 'custom' are introduced. Limitations include child pipelines only supporting custom pipelines, no support for deployments/deployment stages in child pipelines, a maximum of 20 child pipeline steps per parent, and only one level of nesting.