Support Pause/Resume pipeline option

The support to pause and resume a pipeline option is currently not available in UI.

Pausing a pipeline is nothing but creating a PLR with `PipelineRunPending` status and the PLR will not resume its execution until the status is removed from its definition. 

The upstream documentation for pausing (adding PipelineRunPending status) and Resuming (removing the status) a pipeline can be found here.
https://github.com/tektoncd/pipeline/blob/main/docs/pipelineruns.md#pending-pipelineruns