Enhance ParentResourceDescriptionResolver to support nested children and arbitrary keys.

While looking at various subsystems with the intention of integrating with SubsystemResourceDescriptionResolver, I found a number of usage patterns that it does not yet handle:

• Some subsystems use long keys for child resources with several parents (e.g. subsytem.child1.child2.child3). To support this case, SubsystemResourceDescriptionResolver should allow child resolvers to generate other child resolvers.
• Resources of some subsystems use description keys that do not correspond to resource's PathElement. e.g. /subsystem=undertow/setting=http-invoker uses descriptions with keys: undertow.http-invoker (instead of undertow.setting.http-invoker). To support this case, ParentResourceDescriptionResolver should allow creation of a child resolver using an arbitrary key.