rpc: add test-only sendmsgtopeer rpc

This rpc can be used when we want a node to send a message, but
cannot use a python P2P object, for example for testing of low-level
net transport behavior.

src/rpc/client.cpp

@@ -299,6 +299,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "getnodeaddresses", 0, "count"},
     { "addpeeraddress", 1, "port"},
     { "addpeeraddress", 2, "tried"},
+    { "sendmsgtopeer", 0, "peer_id" },
     { "stop", 0, "wait" },
 };
 // clang-format on