Move DecodedServiceEnvelope into its own file (#5715)

src/mqtt/ServiceEnvelope.h

@@ -0,0 +1,13 @@
+#pragma once
+
+#include "mesh/generated/meshtastic/mqtt.pb.h"
+
+// meshtastic_ServiceEnvelope that automatically releases dynamically allocated memory when it goes out of scope.
+struct DecodedServiceEnvelope : public meshtastic_ServiceEnvelope {
+    DecodedServiceEnvelope(const uint8_t *payload, size_t length);
+    DecodedServiceEnvelope(DecodedServiceEnvelope &) = delete;
+    DecodedServiceEnvelope(DecodedServiceEnvelope &&);
+    ~DecodedServiceEnvelope();
+    // Clients must check that this is true before using.
+    const bool validDecode;
+};