Add VecGhostGetGhostIS to get the ghost indices of a ghosted vector

Do not create ISLocalToGlobalMapping in VecGhost by default

The reason is that the global-to-local map is expensive in terms of memory. We only need ghosting indices for our use cases instead of all the indices. Users will still get a complete map if they call VecGetLocalToGlobalMapping. We will not call VecGetLocalToGlobalMapping from MOOSE in order to save memory usage. Created a new API VecGhostGetGhostIS that return ghost indices we need.