Add VDB Point Support to vdb_print

The vdb_print binary is missing support for VDB Point grids. This would be super useful. Some recommended functionality:

* total/active/inactive point count
* attribute name, type, uniform, index, flags, compression information
* group name and count information
* whether the descriptor is shared (option to print out all descriptors which differ?)

I suggest that this is only implemented for ABI=6 and above as there are some new virtual functions that will make this easier.

Start at the vdb_print binary and its existing grid-printing path, then review the ABI=6-and-above point-grid virtual functions mentioned in the issue. Define output for point counts, attribute details, groups, compression, and shared descriptors, including whether differing descriptors are shown. Done means vdb_print reports the agreed VDB Point information for supported ABI versions.

Written by the indexing model from the issue text.