[GR-59661] Further refine CustomGraalClassLoader support for libgraal building.

[graalvmbot]

This PR add the following improvements to the CustomGraalClassLoader support for libgraal building:

1. Specifying a CustomGraalClassLoader is now using a proper, well documented HostedOption instead of a system property. Using native-image --expert-options-detail CustomGraalClassLoader gives

================================
  -H:CustomGraalClassLoader=""
================================
Specify the FQN of the CustomGraalClassLoader implementation that gets used for building libgraal.

This option is only supported for building the libgraal shared library.
The given fully qualified class name has to be a subtype of
jdk.graal.compiler.hotspot.libgraal.CustomGraalClassLoader.

When building the libgraal shared library, this option is used to specify a custom loader
the builder instantiates via default constructor, that affects image building as follows:

 1. The custom loader is used to lookup Feature implementations passed via the --features option.
 2. All @CEntryPoint definitions from classes managed by the custom loader are processed.
 3. All @TargetClass substitutions in classes managed by the custom loader are processed.

2. The builder accesses special custom loader implementation methods via a newly added interface
   jdk.graal.compiler.hotspot.libgraal.CustomGraalClassLoader that an implementation needs to implement. The builder now calls those methods via this interface instead of using reflection.
3. ClassForNameSupport is now again a simple @AutomaticallyRegisteredImageSingleton and ClassForNameSupportFeature is removed again.
4. Instead of providing CustomGraalClassLoader.forEachClass(...) we require a CustomGraalClassLoader.getAllClassNames() method for CustomGraalClassLoader implementations.
5. Some minor cleanups in HostedLibGraalClassLoader.