Add a Leapfrog solver for diffrax

[adrn]

Note that there is an open (stalled) PR to add a similar Verlet integrator to diffrax, but not sure why it hasn't been merged: patrick-kidger/diffrax#303

In the meantime, since we will probably use this quite a bit, we could have an implementation in galax.

cc @jnibauer and @hkotake

[codecov]

Codecov Report

❌ Patch coverage is 88.37209% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.81%. Comparing base (63c120d) to head (256facd).

Files with missing lines	Patch %	Lines
src/galax/dynamics/_src/experimental/leapfrog.py	87.17%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #753      +/-   ##
==========================================
- Coverage   95.87%   95.81%   -0.06%     
==========================================
  Files         158      159       +1     
  Lines        5987     6028      +41     
==========================================
+ Hits         5740     5776      +36     
- Misses        247      252       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.