[FEA]: Add type annotations and export for downstream packages

[TomAugspurger]

Is your feature request related to a problem? Please describe.

I had a bug in my usage of RMM that would have been caught by type annotations (rapidsai/cudf#20154 (comment)).

Describe the solution you'd like

Type annotations for all the public APIs in RMM. This would mean inline annotations in the python files and .pyi files for Cython (IIUC, that's still the best way to do things).

I believe that the packaging needs to be updated to include a py.typed so that downstream projects can use them.

Describe alternatives you've considered
N/A

Additional context
N/A

[bdice]

This is a good idea but I lack bandwidth to implement it at the moment. @TomAugspurger Would you be able to file a PR, even an introductory one with minimal infrastructure so that we can incrementally add typing?

[TomAugspurger]

Yep, I can work on this in the background.

And agreed with the plan of doing this incrementally. Just infrastructure first (maybe with just rmm.statistics as a POC, since that's at least partially typed).