Add TemporaryClasspathExecutor test utility

[scordio]

Overview

This change, initially discussed at #4219 (comment), extracts the logic previously in LauncherFactoryTests to execute code with a custom classpath, which is required for SPI based tests.

The new utility will be used in #4219 to test the new Converter SPI.

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[marcphilipp]

Thank you, @scordio! 👍