Support Pipelines in Pipelines [TEP-0056]

[khrm]

Feature request

We need to add support for TEP-0056 - Pipelines in Pipelines.

Use case

User should be able list all pipelineruns which are embedded as part of PiP.

[khrm]

/assign @khrm

[khrm]

Refer pipeline#7055 and TEP-0056