[JENKINS-62305] Make password parameters work with the build step in Jenkins 2.236+

[dwnusbaum]

See JENKINS-62305.

jenkinsci/jenkins#4630 changed the @DataBoundConstructor for PasswordParameterValue so that one of its parameters is a Secret, but Secret is not supported by data binding via DescribableModel in structs.

This PR uses CustomDescribableModel to maintain compatibility with existing uses of the build step with password parameters, and adds documentation explaining that credentials should be preferred over password parameters going forward.

TODO: Do we want to add a runtime warning (e.g. message in the build log) any time a password parameter is used with the build step? My understanding is that while password parameters are not ideal, they are not completely broken, so I am a little hesitant to add a warning if the issues are relatively minor.

Other things that are broken and that this PR does not attempt to fix:

• Snippet generation for password parameters. I am not sure exactly what is wrong, but the new UI control for passwords from Hide password form fields by default jenkins#3991 seems to be non-interactive in the snippet generator. Additionally, the generated Groovy snippet shows <object of type hudson.util.Secret>, so we'd need to implement customUninstantiate to fix that.
• Doc generation for the build step (https://www.jenkins.io/doc/pipeline/steps/pipeline-build-step/). Once the job that creates those docs is updated to run against Jenkins 2.236 or higher, it will fail. A special case will need to be added to https://github.com/jenkins-infra/pipeline-steps-doc-generator to handle PasswordParameterValue.

[jglick]

Mention also in help-parameters.html.

[dwnusbaum]

FWIW, I realized that snippet generation for password parameters is already broken even on older versions of Jenkins. You can fix it using customUninstantiate, but even if you fix the output this warning is logged, essentially because only BuildTriggerStep implements CustomDescribableModel, not PasswordParameterValue itself:

java.lang.ClassCastException: hudson.model.PasswordParameterValue.value expects class java.lang.String but received class hudson.util.Secret
	at org.jenkinsci.plugins.structs.describable.DescribableModel.coerce(DescribableModel.java:492)
	at org.jenkinsci.plugins.structs.describable.DescribableModel.buildArguments(DescribableModel.java:409)
	at org.jenkinsci.plugins.structs.describable.DescribableModel.instantiate(DescribableModel.java:329)
Caused: java.lang.IllegalArgumentException: Could not instantiate {name=param-name, value=secret} for hudson.model.PasswordParameterValue
	at org.jenkinsci.plugins.structs.describable.DescribableModel.instantiate(DescribableModel.java:334)
	at org.jenkinsci.plugins.structs.describable.DescribableModel.uninstantiate2(DescribableModel.java:672)
	at org.jenkinsci.plugins.structs.describable.DescribableModel.uninstantiate2_(DescribableModel.java:752)
	at org.jenkinsci.plugins.structs.describable.DescribableParameter.uncoerce(DescribableParameter.java:196)
	at org.jenkinsci.plugins.structs.describable.DescribableParameter.uncoerce(DescribableParameter.java:190)
	at org.jenkinsci.plugins.structs.describable.DescribableParameter.inspect(DescribableParameter.java:142)
	at org.jenkinsci.plugins.structs.describable.DescribableModel.uninstantiate2(DescribableModel.java:655)
	at org.jenkinsci.plugins.structs.describable.DescribableModel.uninstantiate2_(DescribableModel.java:752)
	at org.jenkinsci.plugins.workflow.steps.StepDescriptor.uninstantiate(StepDescriptor.java:252)
	at org.jenkinsci.plugins.workflow.cps.Snippetizer.object2Groovy(Snippetizer.java:139)
	at org.jenkinsci.plugins.workflow.cps.Snippetizer.step2Groovy(Snippetizer.java:81)
	at org.jenkinsci.plugins.workflow.cps.SnippetizerTester.assertRoundTrip(SnippetizerTester.java:91)
	at org.jenkinsci.plugins.workflow.support.steps.build.BuildTriggerStepTest.snippetizerRoundTrip(BuildTriggerStepTest.java:588)
	...

Even so, at least snippet generation works with that patch, so I'll push it to the PR branch.