AP_Camera/AP_Mount: add support for external camera gimbal manager

[rmackay9]

ArduPilot already supports the MAVLink Gimbalv2 protocol but it is always essentially the "gimbal manager" (as opposed to the "gimbal device").

Some of the mavlink commands that AP accepts (as gimbal manager) are described on our development wiki

• mount commands
• camera commands

We should also add support for ArduPilot to communicate with another gimbal manager. This would most likely be a program running on a companion computer accepting the same sorts of mavlink messages as ArduPilot does (see above).

Implementing this would likely require writing a new driver (e.g. MNT1_TYPE = GimbalManager). In many cases ArduPilot would just pass through mavlink commands received from the GCS to the gimbal manager but in cases where ArduPilot is the one providing targets it would need to create and send new mavlink messages. This later case includes when the Pilot is attempting to control the camera gimbal using RC or when a mission is being executed that contains camera or gimbal commands.