vmount/navigator/mavlink: gimbal v2 changes

This is a collection of commits all having to do with changes in the
Mavlink gimbal v2 protocol as described in:
https://mavlink.io/en/services/gimbal_v2.html

msg/gimbal_manager_set_manual_control.msg

@@ -0,0 +1,21 @@
+uint64 timestamp						# time since system start (microseconds)
+
+uint8 origin_sysid
+uint8 origin_compid
+
+uint8 target_system
+uint8 target_component
+
+uint32 GIMBAL_MANAGER_FLAGS_RETRACT = 1
+uint32 GIMBAL_MANAGER_FLAGS_NEUTRAL = 2
+uint32 GIMBAL_MANAGER_FLAGS_ROLL_LOCK = 4
+uint32 GIMBAL_MANAGER_FLAGS_PITCH_LOCK = 8
+uint32 GIMBAL_MANAGER_FLAGS_YAW_LOCK = 16
+
+uint32 flags
+uint8 gimbal_device_id
+
+float32 pitch      # unitless -1..1, can be NAN
+float32 yaw        # unitless -1..1, can be NAN
+float32 pitch_rate # unitless -1..1, can be NAN
+float32 yaw_rate   # unitless -1..1, can be NAN