Add package dropping for simulating drone delivery

[Jaeyoung-Lim]

Problem Description
As PX4 vehicles are now being used for package deliveries, this PR adds a package dropping model to simulate a scenario, where a vehicle drops a package that is attached to a parachute.

• A standard_vtol_drop model was added, when one of the aux pins are set over a threshold, releases a package with a parachute.
• The package is dropped by the gazebo_drop_plugin. Any model can be specified through the plugin that the vehicle wants to drop
• A parachute_package model has been added for the parachute attached package.
  

Testing
This was tested with a mavsdk application sending MAV_CMD_DO_SET_ACTUATOR commands.
The example MAVSDK app can be found in PX4/PX4-Autopilot#16758

To run the model,

make px4_sitl gazebo_standard_vtol_drop

Additional Context

• The example MAVSDK app can be found in actuator: add support for MAV_CMD_DO_SET_ACTUATOR PX4-Autopilot#16758
• Corresponding PX4 Firmware PR: Add a payload drop model for drone delivery scenarios in SITL Gazebo PX4-Autopilot#17047