Add filterexact and actearly options to MuJoCo.

When filterexact is a new dyntype which is just like the existing `filter`, but the activation state is integrated with exact integration, instead of simple Euler integration.

When actearly is specified on a general actuator, `qfrc_actuator` is computed using the next timestep's `act` value, instead of the current value.

PiperOrigin-RevId: 554438423
Change-Id: If901e4988fa6b518d6f3097f149770665a189a4b

introspect/structs.py

@@ -2763,6 +2763,13 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  ),
                  doc='bias parameters                          (nu x mjNBIAS)',
              ),
+             StructFieldDecl(
+                 name='actuator_actearly',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjtByte'),
+                 ),
+                 doc='step activation before force             (nu x 1)',
+             ),
              StructFieldDecl(
                  name='actuator_ctrlrange',
                  type=PointerType(