feat: add touch_data to ak.typetracer

[agoose77]

This was requested in #2668, and I've identified that Coffea and dask-awkward currently use our private APIs. We should just make this public.

At this stage, I'd prefer to expose this via ak.typetracer, rather than Content.touch_data, as we can better control the public API (e.g. always recursive, return self), and most people are already working with ak.Array objects.

Closes #2668

[codecov]

Codecov Report

Merging #2672 (0c3411c) into main (6de30f1) will increase coverage by 0.00%.
The diff coverage is 94.73%.

Additional details and impacted files

Files Changed	Coverage Δ
src/awkward/_do.py	83.43% <75.00%> (-0.22%)	⬇️
src/awkward/typetracer.py	94.28% <100.00%> (+1.18%)	⬆️

... and 1 file with indirect coverage changes

[jpivarski]

Great!

I think that Coffea was also using _touch_data temporarily, waiting for some version of it to become public, right @lgray? If so, then this will also serve that purpose.

[lgray]

Yes - there are a few places where we still call _touch_data though length_zero_if_typetracer got rid of most of them.

[danielballan]

Thank you!