Add export option for pipelinerun

[mohitsuman]

The latest tkn support export option for pipelinerun. We need to integrate the workflow to export pipelinerun as yaml and use the yaml to create pipeline run in different namespace.

• Example: export a PipelineRun named 'pipelinerun' in namespace 'foo' and recreate it in the namespace 'bar':

tkn pr export pipelinerun -n foo | kubectl create -f- -n bar

• Output options can be one of:

json|yaml|name|go-template|go-template-file|template|templatefile|jsonpath|jsonpath-as-json|jsonpath-file