[GR-57258] Adapt reflection for multiple layers

[graalvmbot]

This PR allows the reflection metadata to be queried in extension layers to allow registering elements for reflection in extension layers.

There are two different changes:

• ClassForNameSupport is now a MultiLayeredImageSingleton and looks in all knownClasses maps when querying a class name.
• RuntimeMetadataEncoding is also a MultiLayeredImageSingleton and can be queried using a layerNumber. The CodeInfoTable.getFirstImageCodeInfo method also need to fetch the CodeInfo corresponding to a given layer. The layer number is stored in DynamicHub and corresponds to the layer in which the DynamicHub is first created. This means that currently, elements can only be registered for reflection in the layer they are first created.