Add cart-pole demo (backport #289)

[mergify]

Similar to ros-controls/gazebo_ros2_control#172

The difference to gazebo_ros2_control is that it also works with position command interface of the cart. gz_ros2_control does not directly set the position but implements a position controller with velocity output:

gz_ros2_control/gz_ros2_control/src/gz_system.cpp

Lines 632 to 651 in 81dca71

	} else if (this->dataPtr->joints_[i].joint_control_method & POSITION) {
	// Get error in position
	double error;
	error = (this->dataPtr->joints_[i].joint_position -
	this->dataPtr->joints_[i].joint_position_cmd) * *this->dataPtr->update_rate;
	// Calculate target velcity
	double target_vel = -this->dataPtr->position_proportional_gain_ * error;
	auto vel =
	this->dataPtr->ecm->Component<sim::components::JointVelocityCmd>(
	this->dataPtr->joints_[i].sim_joint);
	if (vel == nullptr) {
	this->dataPtr->ecm->CreateComponent(
	this->dataPtr->joints_[i].sim_joint,
	sim::components::JointVelocityCmd({target_vel}));
	} else if (!vel->Data().empty()) {
	vel->Data()[0] = target_vel;
	}

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This is an automatic backport of pull request #289 done by [Mergify](https://mergify.com).