[JENKINS-46124] - support map with EnvStep

[stuartrowe]

Fixes JENKINS-46124

Revival of #105 by @jetersen.

I forked jetersen:fix/JENKINS-46124 and merged the latest from jenkinsci:master.

I also completed the outstanding request to update the help text for the withEnv step and its overrides argument to mention the new map syntax.

Testing done

See mapArguments() and mapArgumentsAsMap() tests added in the original PR.

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[stuartrowe]

@jetersen @dwnusbaum I hope this has been the right approach, but I would really like to see the changes from #105 merged and released 🙏.

[stuartrowe]

@jglick @dwnusbaum @basil is anyone able to review this change please?

[jglick]

Well, I can put it on my backlog, but my backlog is pretty long.