Refactor Cython to use resource_ref instead of device_memory_resource*

[harrism]

As part of the cuda::mr refactoring, eventually there will be no device_memory_resource base class on the C++ side, instead everything will use the relevant type-erased concepts.

In cython, we currently map device_memory_resource onto a cdef DeviceMemoryResource base class that is responsible for keeping the C++ mr alive:

cdef class DeviceMemoryResource:
    shared_ptr[device_memory_resource] mr
    cdef device_memory_resource *get_mr(self)

The get_mr function is used when we need to pass the mr pointer into a C++ API.

There is, in addition, a class hierarchy that builds the various adaptor mrs on top of this (pool, logging, etc...). But this doesn't rely particularly on the existing C++ class hierarchy except that every C++ adaptor mr is-a device_memory_resource (and so can be stored in the same shared_ptr struct slot). This ability goes away once the inheritance disappears on the C++ side.

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1. [FEA] Expose alignment-explicit allocation routines through cython interface #1499
   feature request +1