Introduce a requireSteps step

[batmat]

Some followup of #31 which made me think it would be nice to be more specific. Also, it's sometimes a headache for users to be sure where steps should be coming from.

Sample usage:

requireSteps steps:['pwd','findFiles']

NOTE: this checks only on functions and symbols, not on global variables. @jglick I am not sure how to do that, apart from adding a dependency against groovy-cps which I suspect is probably not the way to go right?

[jglick]

it's sometimes a headache for users to be sure where steps should be coming from

Which is why we need Pipeline Syntax support, which is what I was talking about in #31. And that is not possible until @svanoort finishes jenkinsci/workflow-api-plugin#26.

[svanoort]

@jglick I'm rather unclear why the StepAction is a precursor for pipeline syntax support?

I am only implementing JENKINS-37324, not JENKINS-31582 here -- perhaps there's a mapping from the Descriptor that I'm not aware of?

[HRMPW]

As I said in PR-31, I think the best user experience for this would be to build this into Declarative to be automatic. steps are explicit in Declarative except for anything in a script block. Making a user type in a list of steps they want to use when we already know the list of steps that are being used seems like an unnecessary ask of the Pipeline builder.

If we build this for Scripted and require the user to list the steps included in the Pipeline then that is fine but for Declarative we should automatically fill the list of steps used for the user.

[batmat]

Closing in favor of #31 as per apparent trend above. Cleaning up my open PR list a bit.