control_allocator: implement trim + slew rate limits configuration

msg/CMakeLists.txt

@@ -44,6 +44,7 @@ set(msg_files
 	actuator_motors.msg
 	actuator_outputs.msg
 	actuator_servos.msg
+	actuator_servos_trim.msg
 	actuator_test.msg
 	adc_report.msg
 	airspeed.msg

msg/actuator_servos_trim.msg

@@ -0,0 +1,5 @@
+# Servo trims, added as offset to servo outputs
+uint64 timestamp			# time since system start (microseconds)
+
+uint8 NUM_CONTROLS = 8
+float32[8] trim    # range: [-1, 1]