JVMTI Spec: Add IsModifiableClass to test if class can be redefined/retransformed

JVMTI needs a function to test if a class is modifable with RetransformClasses or RedefineClasses. This will allow an agent to build a list of modifable classes to pass to RetransformClasses. Currently if any of the classes in the list is not modifiable the Retransform (or Redefine) will return an error.

This is partically important in cases where the can_redefine_classes capability is available but the can_redefine_any_class capability is not available (eg: in our implementation where a late-binding agent is loaded into a VM with class data sharing enabled - in that case we can only retransform or redefine shared that aren't shared).