feat: add buffer_key to typetracer_with_report

[agoose77]

This PR makes it possible to format the form_keys assigned to typetracer buffers via ak.typetracer.typetracer_with_report. Although the default behaviour is unchanged, this permits us to ensure that buffer keys are unique within a typetracer layout.

[codecov]

Codecov Report

Merging #2719 (3f40f00) into main (01a1deb) will increase coverage by 0.02%.
The diff coverage is 60.00%.

Additional details and impacted files

Files Changed	Coverage Δ
src/awkward/_nplikes/typetracer.py	77.06% <52.94%> (+0.76%)	⬆️
src/awkward/typetracer.py	87.50% <100.00%> (+0.40%)	⬆️

... and 1 file with indirect coverage changes

[jpivarski]

So, at the time that (public function) typetracer_with_report is called, we assume that users already know what buffer_key function they'll want to use? That does sound reasonable.

Merge when you're ready!