payload_deliverer: Refactor & Handle vehicle command conflicts

Refactor
- Require reboot for PD_GRIPPER_EN parameter change
- Define gripper ACTION_NONE for readability. This makes implicit assumption that -1 equals no-action commanded more explicit
- Tidy the scattered vcmd_ack struct handling cases into a single function
- Refactor to remove return in the middle of function: avoids future complications where a programmer may expect the logic at the end of the function to be executed, but isn't

Vehicle Command Handling
- Cancel the previous running vehicle command if we receive a different vehicle command
- Reject vehicle command if we get a same one that is getting executed
- Save the source system & component of currently running vehicle command
- Added note about new discovered edge case of having same entity sending different gripper commands consequently, where an unexpected ack result may be received

src/modules/payload_deliverer/module.yaml

@@ -10,6 +10,7 @@ parameters:
                 short: Enable Gripper actuation in Payload Deliverer
             type: boolean
             default: 0
+            reboot_required: true
 
         PD_GRIPPER_TYPE:
             description: