- Added passive forces plugins

- Added new `cable` composite type:
  * The `initial` parameter specifies the joint at the starting boundary: `free`, `ball`, or `none`.
  * The boundary bodies are exposed with the names:`B_left` and `B_right`.
  * The vertex initial positions can be specified directly in the XML with the parameter `vertex`.
  * The orientation of the body frame **is** the orientation of the material frame of the curve.

- Added new `cable` passive force plugin:
  * Twist and bending stiffness can be set separately with the parameters `twist` and `bend`.
  * The stress-free configuration can be set to be the initial one or flat with the flag `flat`.
  * New cable example showing the formation of plectoneme.
  * New coil example.
  * New belt example showing interaction between twist and anisotropy.
  * Added test using cantilever exact solution.

PiperOrigin-RevId: 480033694
Change-Id: I491271bce8fccb185961477e903e5a72d172c8a3

unity/Runtime/Bindings/MujocoBindings.cs

@@ -363,6 +363,7 @@ public enum mjtLRMode : int{
 public enum mjtPluginTypeBit : int{
   mjPLUGIN_ACTUATOR = 1,
   mjPLUGIN_SENSOR = 2,
+  mjPLUGIN_PASSIVE = 4,
 }
 public enum mjtGridPos : int{
   mjGRID_TOPLEFT = 0,
@@ -1945,6 +1946,7 @@ public unsafe struct mjModel_ {
   public double* body_inertia;
   public double* body_invweight0;
   public double* body_user;
+  public int* body_plugin;
   public int* jnt_type;
   public int* jnt_qposadr;
   public int* jnt_dofadr;
@@ -2159,6 +2161,7 @@ public unsafe struct mjModel_ {
   public int* sensor_plugin;
   public int* plugin;
   public int* plugin_stateadr;
+  public int* plugin_statenum;
   public char* plugin_attr;
   public int* plugin_attradr;
   public int* numeric_adr;