[FEA] Provide type stubs for pylibcudf package

[wence-]

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

As we build out the pylibcudf API, having type stubs for lsp and type-checking integration becomes increasingly useful.

Describe the solution you'd like

Provide type stubs (with docstrings) so that lsp/type-checker integration works.

We don't want to replicate docstrings in more than one place, I don't know if the right place for them is the type stub file.

Describe alternatives you've considered

Having the relevant pyx file open in an editor at the same time.

Additional context

There are a number of half-working auto-generation engines for stubs, but none of them seem to work that well, so we should probably just do this by hand.

[vyasr]

We may want to look into cython/cython#3818. As @wence- mentioned, many of the existing autogenerators (listed in the linked Cython issue) aren't great, but getting something baked directly into Cython with all of the information Cython already has could in principle work much better.