Add efficient finite-difference Jacobians of mj_step.

- Add `qH` and `qHDiagInv` to `mjData` to save factorized modified inertia.
- Add `mj_EulerSkip`, `mj_implicitSkip`, to `engine_forward.c`.
- Using the above functions, implement `mj_stepSkip` in `engine_derivative.c`.
- Add `mjd_stepFD` and `mjd_transitionFD` to `engine_derivative.c` to compute `mj_step` Jacobians.
  - Exploit "Skip" functionality for speed.
  - Correctly handle quaternion derivatives.
  - Handle warmstarts and control limits.

PiperOrigin-RevId: 456584811
Change-Id: Iee8541f11e7b66feb8f431cb102d9bbe65461f79

include/mujoco/mjxmacro.h

@@ -509,6 +509,8 @@
     X( mjtNum,    efc_aref,          njmax,       1           ) \
     X( mjtNum,    subtree_linvel,    nbody,       3           ) \
     X( mjtNum,    subtree_angmom,    nbody,       3           ) \
+    X( mjtNum,    qH,                nM,          1           ) \
+    X( mjtNum,    qHDiagInv,         nv,          1           ) \
     X( int,       D_rownnz,          nv,          1           ) \
     X( int,       D_rowadr,          nv,          1           ) \
     X( int,       D_colind,          nD,          1           ) \