Moving to PipelineRuns with inline pipeline spec

[michaelsauter]

The current design starts pipeline runs referencing a pipeline. It would also be possible to not maintain pipeline resources at all and only deal with PipelineRuns with an inlined spec. For now this issue only exists to document this idea, without the plan for any immediate action.

Downside:

• The OpenShift console UI allows to navigate from Pipeline to PipelineRun. If we do not have Pipeline resources any longer, there is no good grouping mechanism anymore

Advantages:

• No need to deal with Pipeline resources separately
• We could also inline the task specs (e.g. by reading the spec from an installed task), allowing to give more control to the user (such as overriding compute resources and adding sidecars)

[michaelsauter]

Decided today with @kuebler and @henrjk that we will proceed with this.