Explicitly specify actdim on PID plugin actuators, to allow them to be used with keyframes.

Support actdim attribute on plugin actuators.

Remove the actuator_actdim callback. This callback leads to crashes when loading keyframes for models that have a stateful plugin.

PiperOrigin-RevId: 650649505
Change-Id: I40d1994d02ae4e20ce631b2702a984766cf2628e

introspect/structs.py

@@ -10528,11 +10528,6 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=ValueType(name='int'),
                  doc='number of activation variables',
              ),
-             StructFieldDecl(
-                 name='plugin_actdim',
-                 type=ValueType(name='int'),
-                 doc='actuator state size for plugins',
-             ),
              StructFieldDecl(
                  name='actearly',
                  type=ValueType(name='mjtByte'),