Verifier: Param.Trigger verified against Plan names

[kensipe]

We need to add a verifier That checks that Param.Trigger values are found in the set of the plan names and ERROR if not.

Closely related is: #1618
Prior to this PR, we didn't check trigger when warning that a param is not used... now we need to check if the param.trigger value is sensical value and if not provide an error. The example in verfiy_parameters_test.go is a good example. the newly added {Name: "TRIGGER_ONLY", Trigger: "foo"}, is a trigger on a plan that does not exist. While this may be ok for the current impl... we should have a verifier that errors for trigger that do not exist.

[nfnt]

https://github.com/kudobuilder/kudo/blob/main/pkg/kudoctl/packages/verifier/plan/verify_references.go#L37 does that.
The unit test verify_parameters_test.go doesn't have this check, that's why behavior using non-existing names can be tested in isolation here. The integration tests in package_verify_test.go cover the case where a trigger is set to a non-existing plan.

[kensipe]

woot! thank you