Add mj_extractState as a public API function.

This function allows the caller to extract a subset of components of a state previously obtained via `mj_getState` without having to first write it back into `mjData`.

PiperOrigin-RevId: 822165077
Change-Id: I6433261f4f5ec2e8d024bb7dee8f7dbf1e62666c

python/mujoco/introspect/functions.py

@@ -2416,6 +2416,40 @@ FUNCTIONS: Mapping[str, FunctionDecl] = dict([
          ),
          doc='Get state.',
      )),
+    ('mj_extractState',
+     FunctionDecl(
+         name='mj_extractState',
+         return_type=ValueType(name='void'),
+         parameters=(
+             FunctionParameterDecl(
+                 name='m',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjModel', is_const=True),
+                 ),
+             ),
+             FunctionParameterDecl(
+                 name='src',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjtNum', is_const=True),
+                 ),
+             ),
+             FunctionParameterDecl(
+                 name='srcsig',
+                 type=ValueType(name='unsigned int'),
+             ),
+             FunctionParameterDecl(
+                 name='dst',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjtNum'),
+                 ),
+             ),
+             FunctionParameterDecl(
+                 name='dstsig',
+                 type=ValueType(name='unsigned int'),
+             ),
+         ),
+         doc='Extract a subset of components from a state previously obtained via mj_getState.',  # pylint: disable=line-too-long
+     )),
     ('mj_setState',
      FunctionDecl(
          name='mj_setState',