[JENKINS-46163] Add support for spread and spread-map operators

[dwnusbaum]

See JENKINS-46163. This PR is a copy of cloudbees/groovy-cps#123, which was based on cloudbees/groovy-cps#116.

We follow the same approach as regular Groovy for SpreadMapExpression (the expression gets turned into a special SpreadMap by InvokerHelper.spreadMap, and then Map literals are constructed using InvokerHelper.createMap which detects this marker class and interpolates it).

We use a similar approach for SpreadExpression, but this is not the approach that Groovy itself uses. Some reasoning behind this choice is given in the Javadoc for SpreadBlock.

This PR also includes various improvements to the test harness (565b1ca) which have been copied from groovy-sandbox. I used them in the new tests and adapted existing tests to use the new assertion methods. The two commits can be reviewed independently.

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[jglick]

Do not forget to add release note labels before merging anything.

[jglick]

🤷