Add tkn verify/validate subcommand

[enarha]

Feature request

Add a new subcommand to the tkn cli to verify/validate pipeline syntax. There is a dru-run flag, but it requires k8s cluster or at least credentials. Instead we can de-serialize the yaml file and try to create the Tekton resource, which does not require k8s clusters or credentials. Furthermore we can also test for remote resolutions of referenced objects.

Use case

UI Example