[JENKINS-37227] Metastep support for checkout step - Jenkins Jira

As noted ~~JENKINS-29922~~, we should extend simplified syntax to the checkout step used with SCMs. Proposed task list:

make checkout be a metastep
add a @Symbol to a sample SCM, such as hg on MercurialSCM, and verify that hg can then be run like a top-level step
define an API by which GenericSCMStep/config.jelly could detect that Snippetizer were passing it a particular delegateDescriptor (like MercurialSCM.DescriptorImpl) and switch to an alternate mode in which f:dropdownDescriptorSelector is replaced by the inlined MercurialSCM/config.jelly
investigate whether @Symbol plus tweaks to the parameters on GitSCM and SubversionSCM would allow us to suppress the custom git and subversion step extensions (thus using checkout implicitly) when running on a sufficiently new version of workflow-cps + workflow-scm-step
update WorkflowPluginTest.linearFlow (or .parallelTests) to demonstrate that git now automatically accepts, say, extensions

blocks

JENKINS-39450 Add a LibraryStep for loading shared libraries within a Pipeline

depends on

JENKINS-29922 Promote delegates of metasteps to top-level functions, deprecate $class

is related to

JENKINS-27275 Credentials support for svn step

relates to

JENKINS-44892 Ability to override reflective metadata in DescribableModel

JENKINS-45101 Display meaningful summaries for metasteps

JENKINS-27018 Provide either 'branch' or 'tag' in git step

JENKINS-58503 Symbols for Extensions

(2 relates to)