Custom pointers in container allocators

Hello, I notice that currently some of the containers [don't support allocators which use custom pointer types](https://github.com/abseil/abseil-cpp/blob/04bde89e5cb33bf4a714a5496fac715481fc4831/absl/container/internal/raw_hash_set.h#L756). This could be useful for using the containers with allocators that make use of custom pointers, such as the boost interprocess allocators for working with shared memory. I was wondering if there were any particular reasons it's not supported, and if there were any big issues foreseen with doing so?