Expose reflection to check if casting between two types is supported

[wence-]

Description

In cudf-polars we need to check if a cast between two datatypes is supported (and fallback, or generate different code if not).

Let's ask libcudf to be the source of truth for when a cast is supported.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

Implementation looks good. Having pylibcudf tests would be helpful.

edit: Sorry, this is draft. Maybe I reviewed too early.

[davidwendt]

Approving C++ changes

[mythrocks]

The C++ portion of this was straightforward enough for me to grasp.

I'm not familiar enough with the Python bit, but from a cursory look, the changes seem to track. Thank you for adding tests for this.

[wence-]

/merge