Create data structure to support parent/child pipelines #16094

This issue is meant to be a child of #16094 (closed)

Discussing the topic of parent child pipelines with @ayufan, three steps were identified to implement the feature

1. Create a data-structure in the backend to identify a pipeline as being a child of another
2. Create a filtering mechanism in all appropriate places in the UI to hide child pipelines
3. Decide on the community approved syntax and implement that syntax into the gitlab yaml DSL

This issue pertains to step 1 of the three. In it I will propose potential data structures and outline their pros and cons.