feat: make ak.mean and ak.count differentiable

[Saransh-cpp]

Fixes #2638

Relevant discussion on slack - https://iris-hep.slack.com/archives/C0155BGPGE4/p1707239216196289

cc: @alexander-held

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.94%. Comparing base (b749e49) to head (9626bde). Report is 27 commits behind head on main.

Additional details and impacted files

Files	Coverage Δ
src/awkward/_connect/jax/reducers.py	87.50% <90.00%> (+1.29%)	:arrow_up:

... and 2 files with indirect coverage changes

[agoose77]

@Saransh-cpp is this stuck on anything? Just checking whether I can offer any help.

[Saransh-cpp]

Hi @agoose77, the PR is ready for review; not stuck on anything!