Allow setting JDK path via a java.home JSON setting.

[brown]

Currently, the language server always infers the location of the JDK, either from JAVA_HOME or by searching in various directories that depend on the operating system.

This pull request adds the ability to set the JDK location explicitly via JSON.

[georgewfraser]

Can you add a test @brown ?

[brown]

I would like to add a test, but it doesn't look straightforward. Please suggest an approach you would be happy with. Thanks!

[georgewfraser]

This looks directionally useful, but I don't think it is mergeable yet.

Changes I'd want to see before re-review:

• Make java.home affect the actual JDK model used by the server, not just Docs/src.zip lookup. Right now platform classes still come from the running JVM via ScanClassPath.jdkTopLevelClasses(), so the configured JDK is only partially applied.
• Remove or key the Docs src.zip cache by java.home. As written, the first src.zip lookup can be reused even after the configured JDK changes.
• Add tests that prove a configured java.home changes behavior, not just default src.zip discovery. The current FindSrcZipTest still exercises only the default lookup path.

Happy to re-review once those pieces are addressed.