Add mjs_makeFlex to the MuJoCo C and Python APIs.

- Add mjs_makeFlex C API that wraps mjCFlexcomp::Make(), creating
  a flex with auto-generated bodies, joints, and equality constraints
  from parameters (type, dim, dof, count, cellcount, spacing, scale,
  radius, mass, inertiabox, equality, rigid, flatskin, elastic2d,
  pos, quat, origin, file, vfs).
- Add Python binding body.make_flex() in specs.cc with keyword args.
- Refactor flex_spec_provision.py to use make_flex() instead of XML
  string templating + separate spec + attach. Eliminates textwrap
  dependency.
- Merge user_flexcomp into user_model BUILD target to resolve
  circular dependency.

PiperOrigin-RevId: 922725304
Change-Id: Ice13787c45f3173407fec659f9590a6453bc65dd

include/mujoco/mujoco.h

@@ -1667,6 +1667,15 @@ MJAPI mjsSensor* mjs_addSensor(mjSpec* s);
 // Add flex.
 MJAPI mjsFlex* mjs_addFlex(mjSpec* s);
 
+// Add flexcomp: create flex with auto-generated bodies/joints, return flex spec.
+// Nullable: type, dof, count, cellcount, spacing, scale, pos, quat, origin, file, vfs
+MJAPI mjsFlex* mjs_makeFlex(mjsBody* body, const char* name, const char* type, int dim,
+                            const char* dof, const int count[3], const int cellcount[3],
+                            const double spacing[3], const double scale[3], double radius,
+                            double mass, double inertiabox, int equality, int rigid, int flatskin,
+                            int elastic2d, const double pos[3], const double quat[4],
+                            const double origin[3], const char* file, const mjVFS* vfs);
+
 // Add contact pair.
 // Nullable: def
 MJAPI mjsPair* mjs_addPair(mjSpec* s, const mjsDefault* def);