Support cli command for StepAction CRD

[pratap0007]

Feature request

tektoncd/pipeline has introduced a new CRD, StepAction like Tasks and Pipelines. TKn cli should support some commands for StepAction.

Use case

• List the StepActions
• Describe the StepActions
• Delete the StepAction

UI Example

tkn stepaction list/describe/delete

[pratap0007]

cc: @tektoncd/cli-collaborators