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/mjdata.h

@@ -286,6 +286,10 @@ struct mjData_ {
   mjtNum*   subtree_linvel;       // linear velocity of subtree com           (nbody x 3)
   mjtNum*   subtree_angmom;       // angular momentum about subtree com       (nbody x 3)
 
+  // computed by mj_Euler
+  mjtNum*   qH;                   // L'*D*L factorization of modified M       (nM x 1)
+  mjtNum*   qHDiagInv;            // 1/diag(D) of modified M                  (nv x 1)
+
   // computed by mj_implicit
   int*      D_rownnz;             // non-zeros in each row                    (nv x 1)
   int*      D_rowadr;             // address of each row in D_colind          (nv x 1)