Add cluster and clusterBindings to types

[bobcatfish]

Expected Behavior

The following types should be added:

• pipelineparams should have clusters
• pipelines should have clusterBindings
• tasks should have cluster inputs

(All should be optional)

This wouldn't be expected to actually do anything yet (similar to the rest of the current implementation).

Actual Behavior

We have references to clusters in our example tasks:

https://github.com/knative/build-pipeline/blob/3726a431cee21f9a88bc320339c978cca59a58e5/examples/deploy_tasks.yaml#L17-L18

Clusters defined in our example pipeline params:

https://github.com/knative/build-pipeline/blob/3726a431cee21f9a88bc320339c978cca59a58e5/examples/pipelineparams.yaml#L33-L39

Note that the clusterBinding example is incomplete:
https://github.com/knative/build-pipeline/blob/3726a431cee21f9a88bc320339c978cca59a58e5/examples/pipelines/kritis.yaml#L44-L45

This should be binding a cluster from PipelineParams to a cluster input in the Task, e.g.

          clusterBindings:
              -  inputName: clusterName # refers to the input in the Task
                 clusterName: test # refers to the cluster in the PipelineParms

Additional Info

@barney-s will be working on designs around deployment and k8s cluster interaction for this project

[shashwathi]

/assign @shashwathi