Refactor GpuGenerate expressions/generators

[abellina]

This is a tech debt issue as first mentioned by @revans2 in a PR I put up.

#10131 (comment)_

The request was to move a method inputSplitIndices to the generate expressions themselves. Currently this method is in the GpuGenerator trait.

In general the interface for each trait should be revised, and perhaps split up into other files as well.

We would like to take into account single rows that can explode (for GpuExplode/GpuPosExplode but possibly others) to more than the cuDF column limit, in order to avoid retrying and splitting later: #10131