DSL extension and documentation for native image plugin

[riccardobl]

This PR adds a jmeNativeImage DSL extension to make the GraalVM helper plugin easier to configure.

It also introduces a @PreserveReflection annotation that developers can use to mark their custom classes for inclusion in GraalVM reachability metadata, without needing to configure the Gradle DSL directly.

The plugin still automatically includes subclasses of jME types, such as Savable implementations, network packets, and similar classes. The annotation is mainly intended to support custom reflection use cases that are not covered by the automatic detection logic and would otherwise require the use of targetType in the DSL config.

This PR also adds documentation for the plugin.

[gemini-code-assist]

Code Review

This pull request introduces a new @PreserveReflection annotation and a Gradle DSL extension (jmeNativeImage) to configure GraalVM Native Image reachability metadata for jMonkeyEngine applications. Feedback on the changes includes: 1) in JmeNativeImageExtension.groovy, the proxyInterfaceSet method should use Groovy's toList() instead of the ArrayList constructor to avoid potential runtime exceptions when an Iterable that is not a Collection is passed; and 2) in native-image-metadata.gradle, resource settings should be automatically applied if custom include or exclude patterns are configured, even if useDefaultResourceSettings() is not explicitly called.

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[gemini-code-assist]

The method signature accepts Iterable<String>, but ArrayList does not have a constructor that accepts a raw Iterable (it only accepts Collection). If a user passes an Iterable that does not implement Collection (such as certain Gradle lazy providers or custom iterables), this will throw a MissingConstructorException at runtime.

Using Groovy's toList() extension method on Iterable is safer and more idiomatic.

    void proxyInterfaceSet(Iterable<String> interfaceClassNames) {
        additionalProxyInterfaceSets.add(interfaceClassNames.toList())
    }