common.xml: add a RELAY_STATUS message to report current relay state

message_definitions/v1.0/common.xml

@@ -6195,6 +6195,11 @@
       <field type="uint32_t" name="active" display="bitmask">Active outputs</field>
       <field type="float[32]" name="actuator">Servo / motor output array values. Zero values indicate unused channels.</field>
     </message>
+    <message id="376" name="RELAY_STATUS">
+      <description>Reports the on/off state of relays, as controlled by MAV_VMD_DO_SET_RELAY.</description>
+      <field type="uint64_t" name="time_usec" units="us">Timestamp (since system boot).</field>
+      <field type="uint32_t" name="states" display="bitmask">Relay states.  Relay instance numbers are represented as individual bits in this mask by offset.</field>
+    </message>
     <message id="385" name="TUNNEL">
       <description>Message for transporting "arbitrary" variable-length data from one component to another (broadcast is not forbidden, but discouraged). The encoding of the data is usually extension specific, i.e. determined by the source, and is usually not documented as part of the MAVLink specification.</description>
       <field type="uint8_t" name="target_system">System ID (can be 0 for broadcast, but this is discouraged)</field>